86 PHILOSOPHICAL TRANSACTIONS. [aNNOJ725. 



Observations on the Height of the Barometer, at different Elevations above the 

 Surface of the Earth. By Dr. Nettleton. N° 388, p. 308. 



Having measured a hill of a considerable height, on a clear day, and observed 

 the mercury at the bottom and at the top, it was found according to that esti- 

 mation, that about QO feet, or upwards, were required to make the mercury 

 fall -jij- of an inch ; but coming afterwards to repeat the experiment on a cloudy 

 day, when the air was somewhat gross and hazy, the small angles were so much 

 augmented by refraction, as to make the hill much higher than before, though 

 they were taken carefully with very good instruments, both at that time and 

 before. 



The Doctor afterwards frequently observed at home, by pointing the quadrant 

 to the tops of some of the neighbouring mountains, that they would appear 

 higher in the morning before sun-rise, and also bte in the evening, than at 

 noon, in a clear day, by several minutes: particularly, one morning in Decem- 

 ber, when the vapours lay condensed in the vallies, and the air above was 

 very pure, the top of a mountain, at some distance, appeared more elevated, 

 by above 30', than it had done in the beginning of September about noon, on 

 a very clear day. From whence it appears, that the refraction is at some times 

 greater than at others ; but probably it is always very considerable ; and as there 

 is no certain rule to make allowance for it, it seems likely, that all observations 

 made on very high hills, especially when viewed at a distance, and under small 

 angles, as they commonly are, are uncertain, and scarcely to be depended on, 

 generally erring in making the heights greater than they really are. 



The Doctor then proceeded to observe, as near as possible, the alteration of 

 the mercury in some smaller perpendicular elevations, which we could measure 

 with a line, and also on the tops of some hills of a moderate height, whose 

 altitude could be observed most commodiously, and, by taking the angles large, 

 avoid the danger of any considerable refraction. 



At the bottom of the tower of Halifax church, the mercury stood at 2g.78 

 inch. At the top it subsided to 29.66. The height of the place, where the 

 observation was made, was found to be 102 feet. 



At the bottom of a coal-mine, near this place, the mercury stood at 29.48. 

 At the top, it fell to 29.32. The depth of the mine, being measured, was 

 found to be 140 feet. 



At the bottom of another mine, the mercury was observed to stand at 29. 50. 

 At the top it fell to 29.23. The depth of this mine was 236 feet. 



At the foot of a small hill, whose height could be measured very exactly, 



