A Comparison of the Heat of London and Edinburgh. By John Roebuck, M. D. F. R. S. in a Letter to William Heberden, M. D. F. R. S.

XLIV. A Comparison of the Heat of London and Edinburgh. By John Roebuck, M. D. F. R. S. in a Letter to William Heberden, M. D. F. R. S. SIR, Redde, June 29, 1775. I DELIVERED to you some time ago, a register of the thermometer at Hawkhill for ten years; but as these observations were made at eight o'clock in the morning and four in the afternoon, and yours at eight o'clock in the morning and two in the afternoon, the corresponding years of the morning's observations only admit of a comparison. It appears by your register, that the mean heat at London for nine years, from the end of 1763 to the end of 1772, at eight o'clock in the morning, was $47.4^\circ$; and the mean heat at Hawkhill, during the same period of time, was $46^\circ$. The difference of which is only $1.4^\circ$. A difference much less than might be expected from the difference of latitude, and not sufficient to account why nonpareils, golden rennets, peaches, nectarines, and many kinds of grapes, generally come to maturity near London, and scarce ever near Edinburgh, without the aid of artificial heat. Before I proceeded further to perplex myself with this difficulty, ficulty, I procured from Hawkhill and from yourself the register of the thermometer for three years, at the same periods of time; copies of which I here inclose you. And by these it appears, that the mean heat of London of these three years exceeded that of Edinburgh by $4.5$. And the mean heat of the three hottest months in London exceeded the mean of the same three at Edinburgh by $5.8$. And the mean heat of these three summer months, at two o'clock in the afternoon, in London exceeded the mean heat of the same months, at the same hour, in Edinburgh by $7.3$; which sufficiently accounts why some fruit may come to maturity in one country and not in the other: and also why corn and grapes, which vegetate with a more temperate heat, but require a longer continuance of it, may arrive at maturity in both countries. The reason why the mean heat of London exceeds that of Edinburgh may arise principally from the difference of latitude. But the reason why the excess is greater in proportion in the three hottest months of the year, at the hottest time of the day, than in the winter months, arises from Edinburgh's being situated nearer to the sea than London. We might speak with more precision on this subject, if we had a register of the thermometer at Moscow, which is nearly of the same latitude as Edinburgh; though it is well known, that the heat of summer is much more intense, and the cold of winter much more severe, at Moscow than at Edinburgh. The mean heat of springs near Edinburgh seems to be $47^\circ$; and at Lon- don 51°. It is probable, that the mean heat of good springs in any country is very nearly the mean heat of the country (a). A faithful account of the heat of springs in different latitudes, and of water taken from the same depth of the sea in different latitudes is yet wanted. I am, &c. (a) We shall have an easy method of finding the mean heat of any place, if it be always nearly equal to that of its springs. This matter might be ascertained by a proper number of observations; and it is therefore very desirable, to have an account taken of the heat of the springs, wherever a register is kept of the heat of the air. W. HEBERDEN. | Mean Heat in Pall Mall, London. | |--------------------------------| | | | 1772. | 1773. | 1774. | Mean Heat of Three Years. | |-------|-------|-------|--------------------------| | 8 A.M.| 2 P.M.| 8 A.M.| 2 P.M.| 8 A.M.| 2 P.M.| 8 A.M.| 2 P.M.| | January | 36 | 38 | 42 | 44 | 34 | 39 | 37.3 | 40.3 | | February | 38 | 42 | 36 | 41 | 38 | 44 | 37.3 | 42.3 | | March | 41 | 47 | 40 | 51 | 41 | 52 | 40.7 | 50 | | April | 44 | 51 | 45 | 55 | 47 | 55 | 45.3 | 53.7 | | May | 49 | 60 | 50 | 60 | 51 | 60 | 50 | 60 | | June | 64 | 73 | 58 | 67 | 59 | 67 | 60.3 | 69 | | July | 61 | 72 | 60 | 68 | 61 | 69 | 60.7 | 69.7 | | August | 60 | 70 | 62 | 72 | 62 | 70 | 61.3 | 70.7 | | September | 56 | 65 | 56 | 63 | 55 | 63 | 55.7 | 63.7 | | October | 56 | 61 | 51 | 59 | 48 | 58 | 51.7 | 59.3 | | November | 45 | 55 | 40 | 47 | 40 | 44 | 41.7 | 48.7 | | December | 41 | 44 | 41 | 45 | 39 | 43 | 40.3 | 44 | | Mean | 49.2 | 56.5 | 48.4 | 56 | 47.9 | 55.3 | 48.5 | 56 | Mean heat of three years morning and afternoon was 52.2. Mean heat at HAWKHILL, situated about one mile North of Edinburgh, and 103 feet above the level of the sea. | | 1772 | 1773 | 1774 | Mean Heat of Three Years | |--------|------|------|------|--------------------------| | January| 31.5 | 34.3 | 29.1 | 33.3 | | February| 30.9 | 36.5 | 35.1 | 34 | | March | 37 | 42.8 | 42.1 | 38.7 | | April | 42.9 | 48.5 | 45.6 | 44.2 | | May | 49.1 | 54.5 | 48.6 | 48.1 | | June | 57.2 | 62.1 | 55.2 | 54.5 | | July | 58.7 | 64.6 | 57.7 | 57.9 | | August | 57.4 | 63.9 | 58.3 | 57.6 | | September| 51.5 | 58.1 | 51.3 | 51.5 | | October| 48.8 | 51.6 | 46 | 47.7 | | November| 41.7 | 44.6 | 38.2 | 39.3 | | December| 39.7 | 41.6 | 36.4 | 37.8 | Mean heat, 45.5 50.3 46.1 50.6 44.5 49.5 45.4 50.1 Mean heat of three years morning and afternoon was 47.7.