Astronomical Observations, Made in the Forks of the River Brandiwine in Pennsylvania, for Determining the Going of a Clock Sent Thither by the Royal Society, in Order to Find the Difference of Gravity between the Royal Observatory at Greenwich, and the Place Where the Clock Was Set up in Pennsylvania; To Which are Added, an Observation of the End of an Eclipse of the Moon, and Some Immersions of Jupiter's First Satellite Observed at the Same Place in Pennsylvania: By Charles Mason and Jeremiah Dixon

XLIII. Astronomical Observations, made in the Forks of the River Brandywine in Pennsylvania, for determining the going of a Clock sent thither by the Royal Society, in order to find the Difference of Gravity between the Royal Observatory at Greenwich, and the Place where the Clock was set up in Pennsylvania; to which are added, an Observation of the End of an Eclipse of the Moon, and some Immersions of Jupiter's First Satellite observed at the same Place in Pennsylvania: By Charles Mason and Jeremiah Dixon. Read December 15, 1768. The Place where these Observations were made is the Northernmost Point of the Lines that were measured for a Degree of Latitude, or Point N. (see Tab. XIII. fig. 2.) relative to that Measure; it lies 31 Miles West, by Measurement; and 10°,5 South of the Southernmost Point of the City of Philadelphia, as found by the Sector. | Year | Month | Day | Time per Clock | |------|-------|-----|----------------| | 1766 | Decemb. | 24 | 4 28 40 | | | | | 30 18+ 5 25 3 | | | | | 32 5½ 26 40 | | | | 28 | 4 28 41 | | | | | 30 22+ 5 20 59 | | | | | 32 10 24 30½ | | | | 30 | 4 5 52½ | | | | | 7 12 44 46½ | | | | | 8 34 46 4½ | 1767 January. | Year | Month | Day | Time per Clock | |------|-------|-----|----------------| | | | 4 | 10 33 5 37 45 | | | | 11 | 55 39 10 | | | | 13 | 19½ 40 31- | Equal Altitudes of ditto. | Date | Time per Clock | |------|---------------| | January | | | 7 | 3 50 26 | | | 51 39+ | | | 52 56½ | | 8 | 4 48 | | | 6 7½ | | | 7 30— | | 10 | 6 21 28 | | | 22 43 | | | 24 00 | | 16 | 4 5 4+ | | | 6 25 | | | 7 50 | | 19 | 4 8½ | | | 5 27½ | | | 6 52 | | 27 | 3 32 53 | | | 34 5— | | | 35 19½ | | February | | | 3 | 4 21 12½ | | | 22 22 | | | 23 35+ | | 4 | 3 34 52— | | | 36 5 | | | 37 20 | | 8 | 3 55 32 | | | 56 50+ | | | 58 12 | | 16 | 13 44 50 | | | 13 46 25 | | 25 | 4 11 43— | | | 13 21 | | | 15 7— | | 10 | 42 50— | Equal altitudes of Capella. Equal altitudes of ditto. The first Satellite of Jupiter immersed. Apparent time 8 17 42 47½ Equal altitudes of Castor. The first Satellite of Jupiter immersed. Apparent time 10 12 10 23. Equal altitudes of Capella. Equal altitudes of ditto. Equal altitudes of ditto. Equal altitudes of Capella. Equal altitudes of ditto. The first Satellite of Jupiter was not immersed by clouds. Ditto was immersed. Equal altitudes of Capella. First Satellite of ¼ immersed. Ap. time 25d 12h 24' 40":: From From these observations we have the time of Capella’s passing the meridian, and the rate of the clock’s going as follows: | Year | Month | Pasted merid. per clock | Clock loses of Sid. time per day | Mean state of therm. | Year | Month | Time per watch | |------|-------|-------------------------|----------------------------------|---------------------|------|-------|--------------| | 1766 | Decemb. | 4 57 40+ | 16.3 | 35 | 1764 | March. | 17 8 4 10 Eclipse of the D ended. | | | | | | | | | | | | | 28 56 35 | 18.0 | 23 | | | | | 1767 | January | 55 59 | 13.4 | 6 | | | | | | | 1 55 32+ | 14.8 | 37 | | | | | | | 7 54 3 | 17.0 | 20 | | | | | | | 8 53 46 | 16.3 | 37 | | | | | | | 16 51 36 | 16.0 | 31 | | | | | | | 19 50 48+ | 15.63 | 33 | | | | | | | 27 48 43+ | 15.35 | 28 | | | | | 1768 | February | 4 46 40½ | 15.5 | 30 | | | | | | | 8 45 38½ | 15.9 | 35 | | | | | | | 25 41 8 | | | | | | N. B. The edge of the earth’s shadow on the D’s disk was the best defined I ever saw: it was remarkably distinct from the penumbral shade. N. B. The clock was firmly screwed to a piece of timber, 22 inches in breadth, and five inches and a quarter thick; the said piece of timber was let four feet into the ground, which was composed of a very firm, dry, hard clay. The clock was placed in a tent, with Fahrenheit’s thermometer hung to its side; and a blanket was wrapped round the clock and thermometer, to secure it from any wind that might enter the tent. The pendulum was adjusted to the upper scratch, with No. 3. at the Index, as directed by the Rev. Mr. Maskelyne, Astronomer Royal: but the spring at the suspension of the pendulum having been broke, (when the ship, in which it was sent, was wrecked on the Jersey coast) we cannot be certain that the pendulum is now of the same length as it was when sent from London. Those observations marked : are a little dubious; those marked :: are very dubious; those marked . . * were made per Mr. Dixon. The eclipses of U’s satellites were observed with a reflecting telescope of one foot focus, that magnified about 70 times. | Year | Month | Height of the ther. at about 7h in the mor. in the Tent | Height of the ther. at about 2h in the after. in the Tent | Vibration of the pend. on each side of O., that is, half the arch of vibration. | |------|-------|----------------------------------------------------------|----------------------------------------------------------|-----------------------------------------------------------------------------| | 1766 | Decemb. | 24 .. .. 43 45- | 25 .. .. 44 46 | 26 38 37 45 47 1 40- | | | | 27 38 41 40 42 | | | | | | 28 21 18 31 26 1 35 Near midn. the ther. in the Tent 20 Air 16 | 29 28 28 At 10h P. M. therm. Tent 29 Air 28 | 30 32 32 Near midn. in the Tent 17 Air 14 | | | | 31 5 above O. 3 below O. 18 20 both above O. | | | U u 2 1766 Height of the ther. at about 7h in the mor. in the Tent Air Height of the ther. at about 2h in the aft. in the Tent Air Vibration of the pend. on each side of O. that is, half the arch of vibration. 1766 Decemb. 31 At 8\(\frac{1}{4}\) P. M. ther. in the tent at O. in the air at 7 below O. 1767 At 10\(\frac{1}{4}\) P. M. ditto in the tent at 3 below O. in the air at 13 below O. Jan. 1 At 7h 6' in the morn. the ther. in the \{Tent 10\\ Air 20\} below O. At 9h 45' P. M. ther. in the \{Tent 21\\ Air 17\} above O. Vibration = 1° 12' At 11h 4' P. M. ditto in the \{Tent = 3\\ Air = 12\} below O. Vibration = 1° 10' 2 At 6h 42' in the morn. ther. in the \{Tent 9\\ Air 22\} below O. At 10 ditto vibration of the pendulum = 1° 5' on each side of O. The pendulum now swings a little farther on the west side of O. than on the east side. The clock faces the north. 21 15 1° 7' At 9 P. M. In the \{Tent 9\\ Air 5\} \[ \begin{array}{cccc} 3 & 11 & 9 \\ 4 & before & \circ & rise 34 \\ 5 & 37 & 37 \\ 6 & 49 \\ \end{array} \] \[ \begin{array}{cccc} 39 & 39 & 1° & 20' \\ 48 & 49 & 1 & 35 \\ 53 & 54 & 1 & 40 \\ \end{array} \] \[ \begin{array}{cccc} Pend. now swings rather farthest on the east side of O. \\ At 8\(\frac{1}{2}\) P. M. in the \{Tent 43\\ Air 44\} \end{array} \] 7 At 11h P. M. in the tent 25, in the air 26. 8 28 at 10\(\frac{1}{2}\) P. M. ther. in the tent 23. 9 17 40 40 1° 35' \[ \begin{array}{cccc} Pend. swings 8' more on the east side of O. than on the w. \\ At midn. ther. in the air 25. \end{array} \] 10 43 \[ \begin{array}{cccc} 11 & 50 & 47 \\ \end{array} \] 12 At 4 P. M. in the \{Tent 42\\ Air 44\} 13 42 45 1° 40' 14 23 33 33 15 33 39 41 The pend. swings as on the 9th. | Date | Temperature (°F) | Remarks | |------|------------------|---------| | Jan 16 | 30 30 | Half the arch of vibration. The pendulum swings as before. | | Jan 17 | 43 39 | At 9h 5½ P.M. ther. in the Tent 24 Air 21 | | Jan 18 | 33 31 | At 9h ½ P.M. ther. in the Tent 21 Air 18 | | Jan 19 | 25 26 | | | Jan 20 | 39 40 | | | Jan 21 | 39 39 | | | Jan 22 | 23 21 | | | Jan 23 | 25 23 | | | Jan 24 | 32 32 | Wound up the clock. | | Jan 25 | 32 32 | | | Jan 26 | 28 27 | At 4h ½ P.M. ther. in the Tent 32 Air 32 | | Jan 27 | 21 20 | At 4h ½ P.M. in the Tent 27 Air 25 | | Jan 28 | 11 14 | | | Jan 29 | 15 13 | | | Jan 30 | 16 16 | | | Jan 31 | 32 35 | At 4h ¼ P.M. in the Tent 36 Air 36 | | Feb 1 | 36 35 | | | Feb 2 | 15 13 | | | Feb 3 | 16 15 | | | Feb 4 | 14 10 | | | Feb 5 | 30 32 | | | Feb 6 | 13 12 | | | Feb 7 | 13 12 | | | Date | Height of the ther. at about 7th in the mor. in the Tent Air | Height of the ther. at about 2nd in the aft. in the Tent Air | Half the arch of vibration | |------|----------------------------------------------------------|----------------------------------------------------------|---------------------------| | Feb. 8 | 25 24 | 54 52 | 1° 35' | | At 8h ½ P.M. in the Tent | 33-32 | | Feb. 9 | 32 32 | 42 41 | | Feb. 10 | 41 41 | 34 35 | | Feb. 11 | 25 25 | 40 38 | 1° 40' The pendulum swings as before. | | Feb. 12 | 30 29 | 38 41 | | Feb. 13 | 31 31 | 32 33 | | Feb. 14 | 28 24 | | Feb. 15 | 26 27 | At 4h P.M. in the Tent | 34 | | | | | Air | 33 | | Feb. 16 | 18 10 | 39 48 | | Feb. 17 | 25 17 | 28 28 | | Feb. 19 | 39 44 | | Feb. 20 near noon | tent 46 | air 55 | | Feb. 22 | 14 12 | | Feb. 28 | 69 | 1° 40' Pend. vibrates about 8' farther on the E. side of O., than on the W. side of O., as before. | March | Date | Height | |------|--------| | March 1 | 56 | | March 2 | 46 | | March 3 | 57 | | March 4 | 49 | | March 5 | 51 | | March 6 | 51 | | March 7 | 48 | | March 8 | 56 | | March 9 | 51 | | March 10 | 50 | | March 12 at rise | 26 | | March 13 ditto | 28 | | March 14 | 36 | | March 15 | 47 | | March 16 | 71 | | March 17 | 67 | | March 18 snow | June | Date | Height | |------|--------| | June 4 | 91 | | June 5 | 95 | | June 6 | 95 | The point of the pendulum swings something farther back from the arch (chewing the degrees and minutes) than it did when it was set up. Took down and packed up the clock. Height of the ther. at about 7h in the mor. in the Tent Air Height of the ther. at about 2h in the aft. in the Tent Air | Date | Temperature | |------|-------------| | 7 | 93 | | 8 | 91 | | 9 | 80 | At 4h ½ P.M., 90 at 7h P.M. The air much altered, being very cool and pleasant. N. B. The thermometer is in the shade, and in the same place it was in last winter.