On the Chemical Intensity of Total Daylight at Kew and Pará, 1865, 1866, and 1867

XVII. On the Chemical Intensity of Total Daylight at Kew and Pará, 1865, 1866, and 1867. By Henry E. Roscoe, F.R.S. Received May 14,—Read June 20, 1867. PART I.—THE KEW OBSERVATIONS. In the year 1864 I communicated to the Royal Society* the description of a method for the Meteorological Registration of the Chemical Intensity of Total Daylight, founded upon an exact measurement of the tint which standard sensitive paper assumes when exposed for a given time to the action of daylight. During the last two years measurements of the chemical intensity, according to this plan, have, through the kindness of Dr. Balfour Stewart, been made regularly every day at the Kew Observatory by Mr. T. W. Baker, and thus the practicability of carrying out a continued series of observations according to this method has been effectually and satisfactorily tested. Owing to the press of regular work at the observatory only three separate registrations of chemical intensity could be made at different hours each day. Hence the results obtained do not in any way indicate the hourly variation of chemical intensity, nor can even the individual integrals of daily intensity, giving the mean chemical action each day, be said to do more than exhibit approximately the changes which go on from day to day. The monthly integrals, on the other hand, each calculated from a large number of observations, show with a great degree of accuracy the rise and fall of the chemical intensity with the changing seasons of the year, and enable us to deduce from this the first series of observations of the kind, the mean monthly and yearly chemical intensities at Kew for 1865, 1866, and 1867. The hours at which the chemical intensity was registered each day were those chosen for the reading of the meteorological instruments, viz. 9th 30m A.M., 2nd 30m P.M., and 4th 30m P.M. The condition of the sun’s surface as regards freedom from cloud, the amount of the cloud, the temperature (wet and dry bulb) and the atmospheric pressure were also noted. As an example of the results thus obtained, the observations made in the month of July 1866 have been chosen as exhibiting well the great changes in chemical intensity produced by varying cloud and sunshine. * Bakerian Lecture, Philosophical Transactions, 1865, Part II. p. 605. ### Table I. Observations of Chemical Intensity at Kew, July 1866. | Date | Time | Chemical intensity | Sun's surface | Clouds | Thermometers Fahr. | Barometer at 32° F. | |------|------|--------------------|---------------|--------|-------------------|---------------------| | | | | | | Dry. Wet. | inches | | 1866 | July | 9 30 Rain. | Light clouds. | 8 | 63·7 54·9 | 29·267 | | | | 2 30 0·270 | Clouded over. | 9 | 57·1 51·1 | ·384 | | | | 9 45 0·335 | Id. | 9 | 59·8 54·6 | ·374 | | | | 2 40 0·335 | Light clouds. | 4 | 58·6 54·5 | ·338 | | | | 4 50 0·227 | Clouded over. | 10 | 56·8 53·8 | ·451 | | | | 9 50 0·430 | Id. | 9 | 63·5 55·7 | ·477 | | | | 2 45 0·190 | Id. | 9 | 60·7 55·7 | ·499 | | | | 4 30 0·126 | Unclouded. | 6 | 60·8 53·2 | ·481 | | | | 9 40 0·320 | Id. | 5 | 60·1 56·4 | ·513 | | | | 3 0 0·355 | Clouded over. | 5 | 59·7 52·9 | ·524 | | | | 5 0 0·180 | Rain. | 4 | 60·3 53·7 | ·665 | | | | 9 30 0·187 | Light clouds. | 4 | 60·3 53·7 | ·665 | | | | 4 30 0·187 | Clouded over. | 8 | 57·0 50·1 | ·961 | | | | 9 45 0·122 | Unclouded. | 5 | 63·8 53·7 | 30·013 | | | | 2 0 0·143 | Clouded over. | 10 | 63·6 59·9 | ·179 | | | | 9 40 0·142 | Very light clouds. | 5 | 72·8 65·2 | ·179 | | | | 2 30 0·315 | Id. | 6 | 73·8 66·0 | ·188 | | | | 4 30 0·160 | Unclouded. | 0 | 71·3 64·7 | ·295 | | | | 9 30 0·390 | Hazy. | 0 | 74·8 66·0 | ·265 | | | | 2 40 0·165 | Clouded over. | 9 | 75·5 66·0 | ·249 | | | | 4 30 0·062 | Light haze. | 0 | 75·7 67·5 | ·204 | | | | 9 40 0·560 | Unclouded. | 4 | 81·1 69·2 | ·151 | | | | 2 40 0·630 | Id. | 1 | 88·2 69·5 | ·129 | | | | 5 0 0·237 | Unclouded. | 0 | 78·3 70·5 | ·057 | | | | 9 40 0·640 | Id. | 5 | 79·8 66·5 | ·055 | | | | 4 50 0·250 | Haze. | 4 | 69·2 63·3 | ·141 | | | | 9 30 0·380 | Unclouded. | 4 | 80·6 69·6 | ·125 | | | | 2 0 0·550 | Clouded over. | 5 | 71·8 62·0 | ·044 | | | | 2 50 0·177 | Id. | 10 | 65·9 58·5 | ·050 | | | | 9 40 0·107 | Unclouded. | 9 | 68·1 59·6 | ·012 | | | | 2 40 0·185 | Clouded over. | 10 | 65·9 58·5 | 29·997 | | | | 4 30 0·156 | Id. | 4 | 62·4 55·3 | ·996 | | | | 9 50 0·305 | Unclouded. | 3 | 68·1 56·8 | ·952 | | | | 2 40 0·197 | Id. | 3 | 68·6 57·4 | ·936 | | | | 4 30 0·177 | Clouded over. | 8 | 65·6 59·0 | ·886 | | | | 3 0 0·185 | Id. | 9 | 57·8 50·6 | 30·058 | | | | 9 40 0·257 | Unclouded. | 4 | 65·7 56·8 | ·054 | | | | 2 30 0·355 | Clouded over. | 1 | 68·7 58·3 | ·036 | | | | 4 30 0·203 | Id. | 0 | 64·2 58·2 | ·095 | | | | 9 40 0·177 | Unclouded: very thin haze. | 1 | 75·5 61·2 | ·051 | | | | 2 0 0·345 | Unclouded. | 10 | 56·0 52·3 | ·042 | | | | 9 30 0·217 | Clouded over. | 1 | 65·4 55·7 | 29·985 | | | | 2 40 0·280 | Unclouded. | 1 | 65·7 56·9 | ·986 | | | | 4 30 0·237 | Id. | 10 | 57·8 54·1 | 30·037 | | | | 9 30 0·247 | Clouded over. | 9 | 61·7 53·9 | ·056 | | | | 4 30 0·089 | Id. | 9 | 58·4 53·0 | ·204 | | | | 9 40 0·140 | Unclouded. | 10 | 62·6 55·9 | ·212 | | | | 2 40 0·187 | Id. | 10 | 61·5 55·7 | ·217 | | | | 4 30 0·064 | Id. | 10 | 66·6 58·5 | ·083 | | | | 2 30 0·177 | Id. | 10 | 64·6 57·5 | ·063 | | | | 4 30 0·063 | Id. | 10 | 61·4 59·7 | 29·823 | | | | 9 50 0·063 | Id. | 10 | 63·2 59·8 | ·754 | | | | 2 45 0·092 | Id. | 10 | 64·8 60·8 | ·719 | | | | 4 40 0·088 | Id. | 8 | 65·2 59·7 | ·654 | | | | 9 30 0·193 | Id.; light rain. | 10 | 64·8 61·1 | ·645 | | | | 1 40 0·112 | Rain. | | | | | | | 30. | Rain. | | | | | | | 31. | Rain. | | | | The integrals of daily mean chemical intensity obtained from the numbers in column 3 of the preceding Table by the method described in the above-mentioned memoir, are as follows: **Daily Mean Chemical Intensity.** (Intensity 1·0 acting for 24 hours = 1000.) | Date | Intensity | |---------------|-----------| | July 3rd, 1866 | 138·1 | | „ 4th | 125·1 | | „ 5th | 140·7 | | „ 9th | 91·6 | | „ 12th | 229·0 | | „ 17th | 60·6 | | July 18th, 1866 | 119·4 | | „ 20th | 119·5 | | „ 23rd | 106·6 | | „ 25th | 67·7 | | „ 27th | 35·8 | Monthly mean . . . . 112·2 On the days omitted the number of observations made was too small to enable a determination of daily mean intensity to be made. In a similar manner the daily mean chemical intensity for every day on which a sufficient number of observations were made, has been determined from April 1, 1865, to April 1, 1867. Table II. contains the numbers thus obtained. **Table II.—Daily Mean Chemical Intensities at Kew, 1865–66–67.** (Intensity 1·0 acting for 24 hours = 1000.) | Date | Intensity | Date | Intensity | Date | Intensity | Date | Intensity | Date | Intensity | |------------|-----------|------------|-----------|------------|-----------|------------|-----------|------------|-----------| | 1865. | | 1865. | | 1865. | | 1865. | | 1865. | | | April 1 | 69·2 | May 12 | 105·7 | June 23 | 128·1 | Aug. 24 | 101·3 | Oct. 10 | 38·9 | | 3 | 41·0 | 15 | 40·1 | 26 | 16·7 | 25 | 62·3 | 11 | 29·5 | | 4 | 35·9 | 16 | 99·4 | 27 | 79·5 | 28 | 44·3 | 12 | 37·9 | | 5 | 28·6 | 17 | 60·6 | 28 | 87·2 | 29 | 85·7 | 13 | 19·4 | | 6 | 66·0 | 18 | 129·8 | 29 | 15·2 | 30 | 80·3 | 16 | 18·5 | | 7 | 42·9 | 19 | 109·1 | July 3 | 179·5 | 31 | 70·1 | 17 | 25·9 | | 8 | 96·7 | 22 | 220·8 | 4 | 120·5 | Sept. 1 | 88·1 | 20 | 24·0 | | 10 | 158·2 | 24 | 122·1 | 5 | 73·1 | 6 | 195·5 | 25 | 26·8 | | 11 | 78·1 | 26 | 160·0 | 6 | 103·9 | 7 | 244·7 | Nov. 3 | 14·8 | | 12 | 50·5 | 29 | 115·5 | 7 | 132·0 | 8 | 189·5 | 4 | 16·6 | | 13 | 86·8 | 30 | 100·0 | 10 | 110·3 | 11 | 64·2 | 6 | 12·0 | | 15 | 36·3 | 31 | 64·6 | 11 | 133·3 | 12 | 113·4 | 7 | 9·2 | | 18 | 110·9 | June 1 | 53·1 | 14 | 124·7 | 14 | 129·8 | 8 | 12·0 | | 20 | 73·6 | 2 | 38·0 | 19 | 103·9 | 15 | 165·7 | 9 | 15·7 | | 21 | 125·3 | 6 | 76·0 | 20 | 110·4 | 18 | 113·4 | 10 | 16·7 | | 24 | 82·4 | 7 | 177·0 | 21 | 64·9 | 19 | 75·4 | 11 | 17·6 | | 25 | 87·9 | 8 | 64·6 | 24 | 26·0 | 20 | 102·2 | 13 | 12·9 | | 26 | 89·0 | 9 | 144·5 | 28 | 90·9 | 22 | 97·9 | 15 | 13·9 | | 27 | 106·6 | 12 | 108·7 | Aug. 1 | 46·7 | 25 | 50·7 | 23 | 12·9 | | May 1 | 54·9 | 13 | 135·5 | 4 | 74·0 | 28 | 64·2 | 24 | 12·9 | | 2 | 91·6 | 14 | 96·2 | 8 | 100·0 | 29 | 24·9 | 27 | 13·9 | | 3 | 84·0 | 15 | 68·0 | 14 | 88·1 | Oct. 2 | 44·0 | 30 | 6·5 | | 4 | 59·1 | 19 | 89·7 | 17 | 100·0 | 3 | 34·3 | Dec. 1 | 2·8 | | 5 | 61·1 | 20 | 61·5 | 18 | 74·0 | 4 | 12·9 | 2 | 8·3 | | 8 | 68·8 | 21 | 98·7 | 21 | 137·6 | 5 | 44·5 | 4 | 9·2 | | 9 | 115·9 | 22 | 53·8 | 22 | 114·3 | 6 | 39·8 | 8 | 5·6 | The first result which presents itself from the daily observations is that the mean chemical intensity for hours equidistant from noon is found to be constant; that is, for equal altitudes of the sun the chemical intensities are equal. Thus the mean of all the morning observations in 1865 (207 in number) was at $9^h\ 34^m\ A.M.\ =0\cdot153$; that of the afternoon observations in the same year (197 in number) was at $2^h\ 27^m\ P.M.\ =0\cdot159$; whilst in 1866 the mean of the morning observations (283 in number) was at $9^h\ 49^m\ A.M.\ =0\cdot119$, and the afternoon observations (274 in number) at $2^h\ 29^m\ P.M.\ =0\cdot116$. The morning observations in 1867 (62 in number) at $9^h\ 50^m$ gave $0\cdot044$, the afternoon (58 in number) at $2^h\ 26^m$ gave $0\cdot047$. These give Chemical intensity. Mean of 552 morning observations in 1865–67 at $9^h\ 41^m\ A.M.\ =0\cdot105$ Mean of 529 afternoon observations in 1865–67 at $2^h\ 27^m\ P.M.\ =0\cdot107$ Hence we may with certainty conclude that when the disturbing causes of cloud &c. are eliminated, the daily maximum of chemical intensity corresponds to the maximum of the sun's altitude, and that the chemical intensity exhibits no sign of a post-meridian maximum, as is observed in the measurements of hourly temperature. In order to obtain an expression for the relation existing between the sun's altitude and the chemical intensity of total daylight, a much larger number of observations than the foregoing must be made at widely differing altitudes, either on the same day or on consecutive days. Such a series of observations was made at Heidelberg (see Proceedings Roy. Soc. No. 81, 1866) on a cloudless day. The relation between the sun's altitude and the chemical intensity as found in these determinations is graphically represented in fig. 1A, Plate XXI., and is seen to be a straight line, the abscissæ representing the altitude and the ordinates the corresponding chemical intensity. The formula \[ CI_a = CI_0 + \text{const} \times a \] represents this relation, where \( CI_a \) signifies the chemical intensity at any altitude \( a \) in circular measure, \( CI_0 \) the chemical intensity at the altitude 0, and const. is a number to be calculated from the observations. That this formula closely represents the relation in the case of the Heidelberg observations is seen from the agreement of the observed with the calculated intensities. | Altitude | Observed | Calculated from formula | |----------|----------|------------------------| | 7° 15' | 0.050 | 0.050 | | 24° 43' | 0.200 | 0.196 | | 34° 34' | 0.306 | 0.276 | | 53° 37' | 0.437 | 0.435 | | 62° 30' | 0.518 | 0.506 | A similar series of observations made at Pará (see page 565 of this paper) under a tropical sun in April last, in the middle of the rainy season, shows that a similar relation holds good between the chemical intensity and the sun's altitude even when the sky is not cloudless. | No. of expts. | Sun's mean altitude | Observed | Calculated from formula | |---------------|---------------------|----------|------------------------| | 22 | 7° 34' | 0.964 | 0.959 | | 11 | 6° 40' | 0.769 | 0.800 | | 11 | 4° 28' | 0.685 | 0.666 | | 10 | 2° 58' | 0.344 | 0.338 | This relation is graphically represented in fig. 1B, Plate XXI. Assuming, as we may fairly do, that the same relation between the sun's altitude and chemical intensity holds good at Kew as at Heidelberg and Pará, the value of the inten- sity at noon can be calculated from the observations at 2\textsuperscript{h} 30\textsuperscript{m} and 4\textsuperscript{h} 30\textsuperscript{m} P.M. The observed values of the monthly mean chemical intensities at 9\textsuperscript{h} 30\textsuperscript{m} A.M., 2\textsuperscript{h} 30\textsuperscript{m} P.M., and 4\textsuperscript{h} 30\textsuperscript{m} P.M., from April 1865 to April 1867, are given in Table III.; the values of the intensities at noon have been calculated by help of the foregoing formula. ### Table III. | Month | Hour | Mean intensity | Month | Hour | Mean intensity | Month | Hour | Mean intensity | |-------|------|----------------|-------|------|----------------|-------|------|----------------| | 1865 | April | 9 30 0·195 | 1865 | Dec. | 9 33 0·029 | 1866 | August | 9 34 0·194 | | | | 12 0 0·297 | | | 12 0 0·280 | | | 12 0 0·280 | | | | 2 25 0·215 | | | 2 26 0·210 | | | 2 30 0·210 | | | | 4 38 0·112 | | | | | | 4 42 0·115 | | May | 9 30 0·211 | January | 9 34 0·038 | Sept. | 9 45 0·172 | | | 12 0 0·356 | | 12 0 0·047 | | 12 0 0·286 | | | 2 21 0·240 | | 2 26 0·047 | | 2 32 0·187 | | | 4 30 0·115 | Feb. | 9 39 0·051 | | 4 38 0·058 | | June | 9 33 0·192 | | 12 0 0·094 | October | 9 41 0·085 | | | 12 0 0·313 | | 2 26 0·065 | | 12 30 0·088 | | | 2 26 0·223 | | 4 31 0·021 | | 2 30 0·059 | | | 4 39 0·116 | March. | 9 35 0·081 | | 4 34 0·019 | | July | 9 35 0·218 | | 12 0 0·101 | Nov. | 9 37 0·042 | | | 12 0 0·283 | | 2 30 0·075 | | 12 0 0·057 | | | 2 30 0·214 | | 4 31 0·041 | | 2 27 0·035 | | | 4 30 0·129 | April. | 9 37 0·129 | | 4 21 0·002 | | August| 9 39 0·177 | | 12 0 0·163 | Dec. | 9 43 0·028 | | | 12 0 0·254 | | 2 31 0·116 | | 12 0 0·028 | | | 2 28 0·187 | | 4 43 0·057 | | 2 32 0·016 | | | 4 44 0·104 | May. | 9 37 0·167 | 1867. | January | 9 50 0·033 | | Sept. | 9 39 0·236 | | 12 0 0·259 | | 12 0 0·033 | | | 12 0 0·397 | | 2 28 0·164 | | 12 0 0·033 | | | 2 38 0·271 | | 4 48 0·067 | | 2 31 0·019 | | | 4 35 0·106 | June. | 9 43 0·205 | Feb. | 9 46 0·042 | | October| 9 31 0·066 | | 12 0 0·248 | | 12 0 0·080 | | | 12 0 0·063 | | 2 33 0·183 | | 2 27 0·053 | | | 2 32 0·042 | | 4 43 0·106 | | 4 30 0·012 | | | 4 29 0·013 | July. | 9 38 0·229 | March. | 9 53 0·057 | | Nov. | 9 37 0·046 | | 12 0 0·330 | | 12 0 0·099 | | | 12 0 0·025 | | 2 32 0·238 | | 2 21 0·071 | | | 2 29 0·141 | | 4 39 0·033 | The relations existing between the sun’s altitude and the mean monthly chemical intensities are graphically represented (for 1865) in fig. 2, Plate XXI.; and (for 1866) in fig. 3, Plate XXI. The ordinates denote the intensity, and the abscissae the corresponding altitude of the sun. From the variation in direction of the straight lines representing the relation of intensity to altitude for the different months, it is clear that in each month a different value exists for the constant of the formula, which in fact represents the degree of atmospheric opalescence, the amount of cloud, and the various other factors which, in addition to the sun’s altitude, influence the chemical intensity. That the simple relation which has been shown to hold good when the sun has reached a certain altitude does not apply in the case of low altitudes, is distinctly seen from the above-mentioned figures. When the sun is only a few degrees above the horizon, the disturbing phenomena of opalescence come into play, and the values of the further and yet undetermined terms of the expression become so large as materially to affect the result. It is only in the case of the Heidelberg observations that the first terms of the series express the relation as low as $8^\circ$ of altitude, and this is to be explained by the fact that the observations at Heidelberg were made at an elevation of 1900 feet above the sea-level, and therefore at a situation above a great portion of the denser layers of the atmosphere in which the phenomena of opalescence are most marked. The curves on fig. 4, Plate XXII., show the rise and fall of monthly chemical intensity with the hour of the day for the months of January, February, and March 1867, and April, May, and June 1865; those on fig. 5, Plate XXII., give the same for the last six months of 1865. Figs. 6 and 7 show the same for the twelve months of 1866. The mean monthly integrals of chemical intensity for each month from April 1865 to April 1867, as obtained from these curves, are contained in the first column of the following Table. In the second column are given approximations to this integral obtained by taking the average of the daily means as given on Table II. (pp. 557 and 558). The third column contains the average amount of moisture for the month, in grains per cubic foot; the fourth the relative humidity for the month; the fifth the average amount of cloud at the times of observation; and the sixth the relation between the number of observations on which the sun was overcast (=1) and those made in sunshine. ### Table IV. | Date | Chemical Intensity | Humidity | Cloud | |----------|--------------------|----------|-------| | | I. | II. | III. | IV. | V. | VI. | | 1865 | | | | | | | | April | 97·8 | 77·1 | 3·32 | 0·71 | 4·1 | 1·9 | | May | 117·8 | 98·6 | 3·63 | 0·72 | 6·3 | 0·5 | | June | 82·3 | 83·9 | 4·23 | 0·73 | 4·5 | 1·6 | | July | 114·4 | 105·6 | 4·82 | 0·74 | 6·0 | 1·0 | | August | 88·9 | 84·2 | 4·50 | 0·78 | 6·9 | 0·6 | | September| 107·8 | 114·6 | 4·81 | 0·72 | 2·4 | 3·6 | | October | 23·4 | 30·4 | 3·68 | 0·83 | 4·0 | 1·9 | | November | 17·8 | 13·2 | 3·12 | 0·85 | 6·7 | 0·5 | | December | 8·0 | 2·98 | 0·88 | 7·5 | 0·3 | | 1866 | | | | | | | | January | 15·0 | 15·9 | 2·82 | 0·85 | 6·0 | 0·5 | | February | 24·3 | 24·2 | 2·63 | 0·81 | 6·4 | 0·5 | | March | 34·5 | 30·6 | 2·49 | 0·81 | 5·6 | 0·4 | | April | 52·4 | 49·9 | 3·02 | 0·80 | 6·3 | 0·7 | | May | 78·9 | 70·0 | 2·83 | 0·67 | 5·0 | 0·8 | | June | 92·3 | 86·1 | 4·52 | 0·76 | 6·6 | 1·0 | | July | 106·9 | 111·9 | 4·33 | 0·73 | 6·0 | 0·9 | | August | 94·5 | 95·2 | 4·29 | 0·74 | 7·2 | 0·5 | | September| 70·1 | 100·3 | 4·13 | 0·83 | 6·4 | 0·7 | | October | 29·5 | 40·2 | 3·82 | 0·88 | 6·3 | 0·7 | | November | 15·6 | 17·7 | 2·96 | 0·83 | 5·3 | 0·9 | | December | 14·0 | 3·09 | 0·88 | 6·9 | 0·4 | | 1867 | | | | | | | | January | 13·0 | 8·3 | ......| 0·86 | 7·8 | 0·3 | | February | 21·7 | 17·5 | 2·86 | 0·82 | 7·2 | 0·4 | | March | 30·6 | 27·0 | 2·33 | 0·83 | 7·7 | 0·2 | Although the curves of mean daily chemical intensity showing the variation from hour to hour are symmetrical, the chemical action for hours equidistant from noon being the same, this relation appears by no means to hold good for the curves of yearly chemical intensity. This is distinctly seen if we compare the monthly means for the two months about the vernal with the two about the autumnal equinox, for 1865, 1866, and 1867. | Year | Month | Mean Chem. Int. | |------|-------------|-----------------| | 1865-67 | March 1867 | 30·5 | | | April 1865 | 97·8 | | | September 1865 | 107·8 | | | August 1865 | 88·9 | | 1866 | March 1866 | 34·5 | | | April 1866 | 52·4 | | | September 1866 | 70·1 | | | August 1866 | 94·5 | Or for 100 chemically active rays falling in the months of March and April 1865, 1866, and 1867 at Kew, there fell in the months of September and August 1865–66 167 rays, the sun’s mean altitude being the same in both cases. The curve, fig. 8, Plate XXII., exhibits the biennial variation of chemical intensity at Kew for the two years ending April 1, 1867. The yearly integral for the twelve months January–March 1867 and April–December 1865 is 55·7; whilst that for the twelve months of 1866 is 54·7. The marked differences between the chemical intensities in spring and autumn must be caused by corresponding differences either in the amount of cloud or in the transparency of the atmosphere. From Table IV. (p. 561) it is seen that the mean amount of cloud in March 1867 and April 1865 is 5·9, and that in August and September 1865 = 4·7; whilst the mean cloud for March and April 1866 is 5·9, and that for the corresponding autumn months = 6·8. If the number of observations made when the sun is shining be compared with those made when the sun’s surface is obscured by clouds, it is seen that of sixty-nine observations made in April 1865 the proportion between cloud and sunshine was as 1 to 1·9, whereas in the months of August and September, out of 130 observations the proportion between cloud and sunshine was 1 to 2·1. In 1866 out of 123 spring observations the relation is 1 cloud to 0·55 sunshine, and out of 122 autumn observations the relation was found to be 1 cloud to 0·60 sunshine. Hence it appears that the effect of varying amount of cloud has been eliminated by the number of the observations, and that the difference in chemical intensity cannot be ascribed to the presence of more cloud in the spring than in the autumn. The only other possible explanation is to be sought in the difference in atmospheric transparency in spring and autumn, and the only indication which we at present possess of such variation in transparency is afforded by measurements of the hygrometric condition of the air, the increased transparency of moist air for the visible rays being well known. In March 1867 and April 1865 the mean amount of moisture was found to be 2·82 grains per cubic foot; in August and September 1865 it was 4·65 grains. In March and April 1866 the moisture was 2·8 grains, and in August and September 4·21 grains. This gives a relation of 1 to 1·65 for spring and autumn moisture 1865, and 1 to 1·50 for the same in 1866. Another important factor as influencing the transparency must not be overlooked, viz. the presence of finely divided solid particles which floating about produce the phenomenon of atmospheric opalescence. This, taken in connexion with the well known fact of the greater velocity of the winds in spring than in autumn, thus increasing the quantity of these floating particles in the spring, points to an explanation of the high autumnal and low vernal chemical intensity. **Part II.—The Pará Observations.** Our knowledge concerning the distribution of the chemically active rays on the earth's surface is as yet very limited, and any conclusions with respect to the intensity of the chemical rays in the tropics have been, up to the present time, based upon the vague and unsatisfactory statements of photographers. According to photographic observations it would appear that in advancing from England towards the equator the difficulty of obtaining good pictures is increased, and more time is said to be required to produce the same effect upon a sensitive film under the full blaze of a tropical sun than in the gloomier atmosphere of London*. It is likewise stated† that in Mexico, where the light is very intense, from twenty minutes to half an hour was required to produce photographic effects which in England occupy but a minute; and it is said that travellers engaged in copying the antiquities of Yucatan have on several occasions abandoned the use of the photographic camera and taken to their sketch-books. Dr. Draper has also noticed certain differences of a similar kind between the light of New York and that of Virginia, and hence a supposition has been thrown out of the existence of a peculiar retarding action exerted by the luminous and the calorific rays upon the more refrangible and chemically active portion of the sunlight. In order to test the validity of these statements, it becomes a matter of great interest to determine directly the intensity of the chemically active rays in the tropics. Through the kindness of Messrs. Alfred Booth and Co., of Liverpool, and thanks to the zeal and ability of my assistant, Mr. T. E. Thorpe, I have been able to obtain such a set of measurements made at Pará, situated nearly under the equator in the northern province of the Brazils, and lying upon a branch of the Amazons, in longitude 48° 30' West, and latitude 1° 28' South. The observations, the results of which are given below, were made at Pará by Mr. Thorpe from the 4th to the 26th of April 1866, in a situation possessing a clear horizon. Owing to the rainy season having set in when the experiments were commenced, the changes in the chemical intensity as observed from hour to hour, and even from minute to minute, are very sudden and remarkable, and render a large number of daily observations necessary. These sudden changes are well represented in the curves, figs. 9, 10, 12, and 13, Plate XXI., showing the variation of chemical intensity at Pará during the * Golding Bird, 'Natural Philosophy,' p. 622, 5th Edit. † Robert Hunt, 'Researches on Light,' p. 366. days of April 18th, 23rd, 25th, and 26th. The curves for these days, compared with the dotted lines below, indicating the corresponding action at Kew, show the enormous variation in chemical intensity which occurs under a tropical sun in the rainy season. Regularly every afternoon, and sometimes at other hours of the day, enormous thunder-clouds obscure the sun, and discharging their contents in the form of deluging rain, reduce the chemical action nearly to zero. The storm quickly passes over and the chemical intensity rapidly rises to its normal value. If we compare the daily mean intensities at Pará and Kew on the same days, we gain some idea of the true chemical action of the tropics, and it becomes at once evident that the alleged failure of photographers cannot, at any rate, be ascribed to a diminution in the sun’s chemical intensity, but must rather be referred either to overexposure of the plate, or more probably to the difficulty of obtaining a distinct image owing to constant variation in the density of the layers of air intervening between the plate and the object. The curves, figs. 9 to 14, Plate XXI., exhibit graphically the relation of chemical intensity at Kew and Pará on the 18th, 23rd, 24th, 25th, and 26th of April 1866, these being chosen from the other sets of observations as being the most complete. The data for these five days’ observations are found in the Tables at the end of this paper. The following numbers give the Daily Mean Chemical Intensities at Kew and Pará for fifteen days in April 1866. | Date | Daily mean Intensity | Ratio | |------|---------------------|-------| | | Kew. | Pará. | | | 1866.| | | | | April| 4... | ...... | 269·4 | | | 6... | 28·6 | 242·0 | 8·46 | | | 7... | 7·7 | 301·0 | 39·09 | | | 9... | 5·9 | 326·4 | 55·25 | | | 11... | 25·4 | 233·2 | 9·18 | | | 12... | 55·8 | 203·1 | 3·66 | | | 13... | 52·2 | 337·8 | 6·46 | | | 14... | 38·5 | 265·5 | 6·89 | | | 18... | 39·8 | 350·1 | 8·80 | | | 19... | 75·2 | 352·3 | 4·68 | | | 20... | 38·9 | 385·0 | 9·90 | | | 23... | 80·4 | 350·1 | 4·35 | | | 24... | 83·6 | 362·7 | 4·34 | | | 25... | 73·7 | 307·8 | 4·17 | | | 26... | 39·1 | 261·1 | 6·67 | Hence it appears that the chemical action of total daylight in the month of April 1866 was 6·58 times as great at Pará as at Kew. In order to form an idea of the march of the daily chemical intensity under the equator in the sunshine, all the observations made when the sun’s disk was unobscured by clouds have been collected, and a curve plotted out from the means thus obtained. The following Table gives the results, and the curve, fig. 14, Plate XXI., exhibits the regular nature of the increase before and after noon. The curve is a symmetrical one, and exhibits a maximum at noon; the dotted curve is the curve of mean chemical intensity for April at Kew, and the relation between these two intensities is as 52·4 to 313·3, or a ratio of 1 to 5·98. | Mean time | No. of observations | Intensity | |-----------|---------------------|-----------| | h m | | | | 7 3 | 11 | 0·196 | | 7 54 | 11 | 0·389 | | 9 24 | 8 | 0·789 | | 10 1 | 19 | 0·871 | | 11 5 | 27 | 0·983 | | 12 1 | 21 | 1·028 | | Mean time | No. of observations | Intensity | |-----------|---------------------|-----------| | h m | | | | 12 54 | 13 | 0·981 | | 2 5 | 17 | 0·820 | | 2 54 | 14 | 0·664 | | 3 57 | 7 | 0·406 | | 4 49 | 4 | 0·223 | In a future communication I propose to discuss the relation between the chemical intensity of direct and diffuse sunlight at Kew, Pará, and Lisbon. Chemical Intensity of Total Daylight at Pará, April 12th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C. | Barometer. | |------------|--------------------|-------------------------|--------|-----------------|------------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 9 30 | 0·348 | Clouded; dull. | 9–10 | 28·1 | 25·7 | 764·4 | | 9 45 | Rain. | | | | | | | 9 55 | 0·731 | | 9–10 | 28·4 | 26·4 | | | 10 0 | Rain. | | | | | | | 10 25 | 0·947 | Unclouded. | 8 | 29·4 | 28·3 | | | 10 35 | Rain. | | | | | | | 10 55 | Id. | | | | | | | 11 2 | 0·971 | Unclouded. | 7–8 | 29·4 | 27·3 | | | 11 30 | 1·019 | | 7 | 31·1 | 27·2 | | | 11 55 | 1·019 | Unclouded. | 5 | 30·1 | 25·9 | 764·2 | | 1 14 | 0·968 | Id. | 6 | 27·8 | 26·1 | | | 2 20 | …… | Very heavy rain. | …… | …… | …… | 763·0 | | 2 45 | 0·744 | | 10 | 27·8 | 26·6 | | | 3 0 | 0·190 | Gloomy; thunder. | 10 | 26·6 | 25·0 | | | 3 13 | Rain. | | | | | | | 4 30 | …… | Heavy rain. | | | | | ### Chemical Intensity of Total Daylight at Pará, April 13th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 7 13 | 0·336 | Unclouded. | 2 | 24·6 | 34·0 | 763·5 | | 8 0 | | | 3 | 25·8 | 24·4 | | | 9 30 | 0·851 | | 3 | 29·1 | 25·9 | | | 10 0 | | | | | | | | 10 20 | | | | | | | | 10 45 | 0·565 | Clouded. | 7–8 | 30·0 | 25·9 | | | 11 10 | 0·570 | | | 30·0 | 26·1 | 765·0 | | 11 20 | | Rain. | 8 | 30·5 | 26·7 | | | 11 35 | 1·079 | Sunshine. | | 31·4 | 27·6 | | | 11 50 | 0·980 | | 7–8 | 31·7 | 27·2 | | | 12 10 | 0·665 | Clouded. | | 32·0 | 27·2 | | | 12 37 | 0·474 | | | 30·9 | 27·2 | | | 1 0 | 1·080 | Sunshine. | 7 | 30·4 | 26·9 | | | 1 10 | | Rain. | 8 | | | | | 1 42 | 0·210 | Clouded. | 8–9 | 26·8 | 25·6 | | | 1 45 | | Thunderstorm. | | | | | | 2 23 | 0·425 | Sunshine cloud. | 8 | 26·9 | 25·1 | 762·0 | | 2 43 | 0·743 | Unclouded. | 7–8 | 28·2 | 25·9 | | | 3 2 | 0·420 | Sunshine cloud. | | 28·3 | 25·6 | | | 3 15 | 0·378 | | 8 | 28·1 | 25·3 | | | 3 37 | 0·248 | Clouded. | 8 | 27·9 | 25·3 | | | 4 0 | | Heavy rain. | | | | | ### Chemical Intensity of Total Daylight at Pará, April 19th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 6 50 | 0·227 | Clouded. | 5 | 24·5 | 24·3 | 766·5 | | 7 15 | 0·333 | Id. | 5 | 25·0 | 24·4 | | | 7 40 | 0·360 | Id. | 5 | 25·6 | 25·0 | | | 8 0 | 0·416 | Id. | 9–10 | 26·6 | 25·6 | | | 9 25 | 0·850 | Unclouded. | 6–7 | 28·4 | 26·0 | | | 9 49 | 0·839 | Id. | 6–7 | 30·6 | 27·4 | | | 9 52 | 0·803 | Id. | 6–7 | 30·6 | 27·4 | | | 10 30 | 0·791 | | 6–7 | 30·9 | 27·3 | | | 10 47 | 1·266 | Unclouded. | 4–5 | 31·8 | 27·6 | 765·0 | | 10 49 | 1·115 | Id. | 4–5 | 31·8 | 27·6 | | | 11 25 | 0·900 | Id. | 5 | 32·6 | 27·6 | | | 11 27 | 1·050 | Id. | 5 | 32·6 | 27·6 | | | 12 41 | 0·940 | Id. | 3–4 | 33·3 | 27·8 | | | 1 50 | 0·564 | Clouded. | 6–7 | 29·1 | 26·3 | | | 2 15 | 1·000 | | 6–7 | 28·1 | 25·6 | 762·5 | | 2 46 | 0·739 | | 9 | 29·4 | 26·3 | | | 3 10 | | Heavy rain. | | | | | | 3 30 | 0·260 | Clouded. | 9–10 | 26·2 | 25·3 | | | 3 50 | | Rain. | | | | | ### Chemical Intensity of Total Daylight at Pará, April 20th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 7 40 | 0·456 | Unclouded. | 1 | 26·1 | 25·0 | 764·5 | | 9 53 | 0·940 | Id. | 1 | 29·4 | 26·1 | | | 10 21 | 1·000 | Id. | 3 | 30·8 | 27·1 | | | 10 48 | 0·768 | Id. | 3–4 | 31·7 | 27·5 | | | 11 31 | 0·893 | Id.; hazy. | 4–5 | 32·2 | 27·2 | | | 12 0 | 0·900 | Id.; id. | 4–5 | 34·1 | 28·1 | | | 12 32 | 0·960 | Id. | 4–5 | 34·3 | 27·8 | | | 1 2 | 0·908 | Id. | .......| 33·7 | 28·2 | | | 3 5 | 0·336 | Clouded; gloomy. | 8 | 31·9 | 27·2 | | | 3 16 | 0·237 | Id.; id. | 8–9 | 31·3 | 27·5 | | | 3 40 | 0·539 | .........................| 7–8 | 29·4 | 26·4 | | | 4 0 | 0·452 | Unclouded. | 7–8 | 27·8 | 25·7 | | | 4 20 | 0·333 | Clouded. | 7–8 | 28·4 | 26·1 | | ### Chemical Intensity of Total Daylight at Pará, April 23rd, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 9 5 | 0·761 | .........................| 7 | 28·1 | 25·9 | 766·5 | | 9 15 | 0·532 | Clouded. | 8 | 28·1 | 25·8 | | | 9 30 | 1·079 | .........................| 6 | 28·4 | 26·1 | | | 9 45 | 0·725 | Clouded. | 5 | 29·4 | 26·5 | | | 10 0 | 1·402 | Unclouded. | 4 | 29·6 | 26·5 | | | 10 15 | 1·019 | .........................| 5 | 29·4 | 26·2 | | | 10 30 | 1·105 | Unclouded. | 4 | 30·6 | 26·4 | | | 10 45 | 1·114 | Id. | 4 | 29·6 | 27·0 | | | 11 0 | 1·148 | Id. | 4 | 31·6 | 27·2 | | | 11 17 | 1·318 | Id. | 4 | 32·9 | 27·4 | | | 11 30 | 0·674 | Clouded. | 5 | 32·5 | 27·2 | | | 11 45 | 1·019 | Unclouded. | 3 | 32·8 | 26·1 | | | 12 0 | 1·019 | Id. | 3 | 32·8 | 26·1 | | | 12 15 | 1·054 | Id. | 3–4 | 32·2 | 25·7 | | | 12 30 | 1·344 | Id. | 4 | 32·1 | 25·3 | | | 12 45 | 0·689 | Clouded. | 8 | 31·7 | 25·4 | | | 1 0 | Rain. | | | | | | | 1 12 | 0·444 | Clouded. | 8 | ...............| ...........| 764·5 | | 1 30 | 1·002 | Unclouded. | 4 | 29·0 | 26·1 | | | 1 50 | 0·874 | .........................| 7 | 30·0 | 26·1 | | | 2 5 | 0·925 | .........................| 7 | 30·0 | 26·1 | | | 2 15 | 0·968 | .........................| 7–8 | 30·6 | 26·6 | | | 2 30 | 0·925 | .........................| 7–8 | 30·9 | 26·6 | | | 2 45 | 0·977 | .........................| 7–8 | 31·1 | 26·6 | | | 3 0 | 0·856 | Unclouded. | 6 | 30·9 | 26·6 | | | 3 15 | 0·280 | Overcast; gloomy. | 8–9 | 28·9 | 26·1 | | | 3 30 | 0·384 | Id.; id. | 9–10 | 27·8 | 25·7 | | | 3 45 | 0·352 | Overcast. | 9–10 | 27·2 | 25·1 | | | 4 0 | Rain. | | | | | | | 5 0 | 0·233 | Overcast. | 8–9 | 25·7 | 24·8 | | | 5 10 | 0·200 | Id. | 8 | 25·7 | 25·0 | | | 5 20 | Rain. | | | | | | ### Chemical Intensity of Total Daylight at Pará, April 24th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 6 55 | 0·151 | Sunshining through mist.| ...... | 24·4 | 24·2 | 763·5 | | 7 30 | 0·213 | Clouded. | 9–10 | 25·6 | 24·9 | | | 8 0 | 0·359 | Id. | 8 | 26·6 | 25·6 | | | 9 31 | 0·633 | Thin haze. | 2 | 29·4 | 26·4 | | | 10 2 | 0·684 | Unclouded. | 2 | 30·0 | 26·4 | 767·0 | | 10 30 | 0·719 | Id. | 2 | 30·6 | 26·1 | | | 11 3 | 0·951 | Id. | 3 | 32·3 | 27·8 | | | 12 0 | 1·019 | Id. | 4 | 31·7 | 25·9 | | | 1 | 0·942 | Id. | ...... | 31·7 | 25·3 | | | 2 0 | 0·754 | Id. | 4 | 32·2 | 26·1 | 764·5 | | 3 0 | 0·492 | Id. | 2 | 32·2 | 25·1 | | | 3 51 | 0·389 | Id. | 3 | 30·1 | 26·6 | 762·5 | | 4 29 | 0·306 | Id. | 3–4 | 29·0 | 26·2 | | ### Chemical Intensity of Total Daylight at Pará, April 25th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | | | | | h m | | | | | | | | 6 48 | 0·116 | Unclouded. | 0·5 | 24·2 | 23·6 | 765·5 | | 7 31 | 0·312 | Id. | 0·5 | 25·9 | 25·0 | | | 9 41 | 0·490 | Clouded over. | ...... | 29·4 | 25·4 | 766·8 | | 10 3 | 0·762 | Unclouded. | 3–4 | 31·3 | 27·0 | | | 10 29 | 0·944 | Id. | 5 | 31·9 | 26·5 | | | 10 53 | 0·529 | Clouded over. | 4–5 | 31·1 | 26·2 | | | 10 55 | 0·959 | Unclouded. | 4–5 | 31·1 | 26·2 | | | 11 29 | 0·475 | Clouded over. | 6–7 | 32·7 | 27·4 | | | 11 30 | 0·976 | Unclouded. | 6–7 | 32·7 | 27·4 | | | 11 45 | 0·479 | Clouded over. | 5 | 32·2 | 26·6 | 766·0 | | 12 0 | 1·011 | Unclouded. | 5–6 | | | | | 12 16 | 0·977 | Id. | 5 | 32·0 | 26·1 | | | 12 47 | 0·882 | .........................| ...... | 31·3 | 26·9 | | | 1 0 | 0·335 | Overcast; gloomy. | ...... | 29·7 | 26·6 | | | 1 23 | 0·365 | Id.; id. | ...... | 29·7 | 26·7 | | | 1 47 | 0·774 | Unclouded. | 5 | 30·8 | 27·5 | | | 2 39 | 0·236 | Clouded over. | 8 | 28·3 | 26·1 | | | 2 45 | Rain. | | | | | | | 3 6 | 0·677 | Unclouded. | 7–8 | 29·7 | 27·2 | 763·0 | | 3 45 | Rain. | | | | | | | 3 49 | 0·210 | Clouded. | 9 | 28·3 | 26·1 | | | 4 5 | Rain. | | | | | | Chemical Intensity of Total Daylight at Pará, April 26th, 1866. | Solar time | Chemical intensity | Condition of solar disk | Clouds | Temperature °C | Barometer | |------------|--------------------|-------------------------|--------|----------------|-----------| | | | | | Dry. | Wet. | millims. | | h m | | | | | | | | 7 43 | 0·360 | Unclouded. | 2 | 26·3 | 25·3 | 766·0 | | 8 19 | 0·408 | Clouded. | 7–8 | 26·7 | 25·0 | | | 10 0 | 0·958 | | 7–8 | 30·8 | 26·6 | 767·0 | | 10 15 | 0·354 | Clouded; gloomy. | 10 | 29·4 | 26·1 | | | 10 30 | 0·608 | Id.; id. | 9 | 28·9 | 25·6 | | | 10 45 | 0·650 | Id. | 9 | 30·0 | 26·0 | | | 11 0 | 0·822 | | 8–9 | 29·9 | 26·4 | 766·5 | | 11 15 | 1·037 | | 7–8 | 31·7 | 27·2 | | | 11 30 | 1·088 | Unclouded. | 7 | 32·2 | 27·5 | | | 11 45 | 1·011 | Id. | 5 | 31·1 | 25·6 | | | 12 0 | 0·539 | Clouded. | 5–6 | 30·8 | 25·0 | 765·5 | | 12 10 | 1·036 | Unclouded. | 3–4 | 31·0 | 25·5 | | | 12 30 | 0·976 | Id. | 3 | 32·2 | 26·6 | | | 1 20 | 0·831 | Id. | 5 | 32·3 | 25·8 | | | 2 13 | 0·608 | Id. | 5 | 31·9 | 25·0 | | | 2 33 | 0·540 | Id. | 6–7 | 29·7 | 26·4 | 763·8 | | 2 53 | 0·336 | Clouded. | 8 | 27·4 | 24·7 | | | 3 0 | Rain. | | | | | | | 3 30 | Rain. | | | | | | | 3 34 | 0·200 | | 8–9 | 26·3 | 25·3 | 763·5 | | 3 46 | 0·166 | Clouded. | 8–9 | 26·3 | 25·3 | |