VOL. LXXVm.] PHILOSOPHICAL TRANSACTIONS. 405 



variations entirely regulated by the clearness, or the cloudiness, of the sky; and 

 though they did not always happen in the same proportion to the respective 

 altitudes, yet, when the thermometers differed at all, that on the ground was 

 always the coldest. 



Finding so considerable a difference as 3-i-° within 6 feet of the earth's surface, 

 Mr. S, increased the number of thermometers in the meadow to 4 ; one of them 

 he sunk in the ground, another he placed just on the ground, a third he sus- 

 pended at 3 feet, and a fourth at 6 feet from the ground. At the same time he 

 placed 3 thermometers in an open garden on St. Thomas's Hill, where the land 

 is level with the Cathedral Tower, and about a mile distant from it; here he like- 

 wise put one in the ground, another just on it, and suspended a third 6 feet 

 above it. With these 7 thermometers and the 2 before-mentioned, in the city, 

 he continued a diary for 20 days, taking also every morning the temperature of 

 the water in the river ; but the weather proving cloudy soon after, the thermo- 

 meters hardly varied at all, 7 or 8 days only excepted. After this time he never 

 Rectified them but when the appearance of the weather gave reason to expect that 

 they would vary considerably : by which it appears, that the cold in the night was 

 generally greater in the valley than that on the hill ; but that the variations be- 

 tween the thermometers on the ground, and those 6 feet above them, were often 

 as great on the hill as in the valley. 



From the foregoing experiments it appears, that a greater diminution of heat 

 frequently takes place near the earth in the night-time, than at any elevation in 

 the atmosphere within the limits of Mr. Six's inquiry; and that the greatest 

 degrees of cold are at such times always found nearest to the surface of the earth; 

 that this is a constant and. regular operation of nature, under certain circum- 

 stances and dispositions of the atmosphere, and takes place at all seasons of the 

 year ; that this difference never happens in any considerable degree but when the 

 air is still, and the sky perfectly unclouded ; but the moistest vapour, such as 

 dews and fogs, did not, as far as he could perceive, at all impede, but rather in- 

 crease the refrigeration. In very severe frosts, when the air frequently deposits a 

 great quantity of frozen vapour, he generally found it greatest ; but the excess 

 of heat, which in day-time, in the summer season, was found at the lower sta- 

 tion, in the winter diminished almost to nothing. 



The foregoing experiments related to the difference of heat which, at certain 

 times, is found at different altitudes ; the following to the different degrees of 

 heat observed at different situations in respect to the sea-shore. Mr. S. exhibits a 

 set of corresponding observations ; among which are some taken at Chislehurst, 

 by the Rev. Mr. Wollaston ; others at the same time were taken in Mr. S.'s 

 garden, and on the Cathedral Tower; and others on the sea-shore, about 7 miles 

 N. N. w. from Canterbury, where the thermometer was suspended about 40 feet 



