VOL. XXXVI.] PHILOSOPHICAL TRANSACTIONS. 331 



circumambient air, when it is not confined; and the density and rarity of the 

 air is chiefly owing to the increased or diminished pressure, by the accumula- 

 tion or exhaustion of superior air, while heat and cold alter its density in a much 

 less proportion ; the clouds made of the vapours abovementioned, instead of 

 conforming themselves to the altered density of the ambient air, will rise when 

 it is condensed, and sink when it is rarefied; and also rise or sink, when the 

 pressure of the air is not altered, and its density very little changed, by their 

 own dilatation, owing to heat or cold ; as may be observed often, by seeing them 

 change their height considerably, while the barometer continues exactly at the 

 same degree, and the thermometer's liquor rises or falls very little, and some- 

 times not at all. 



As for the manner how clouds are changed into rain, the Doctor has hinted 

 it in the beginning of this paper: but for further satisfaction, he refers the 

 reader to Dr. Halley's account of it, in the Phil. Trans. N° ] 83, in which Dr. 

 D. entirely acquiesces, having always found it agreeable to the phaenomena. 



Since the Doctor has, for brevity sake, only mentioned at what heights from 

 the surface of the earth, vapours of different densities will come to an equili- 

 brium, without giving a reason for settling the place of equilibrium, at those 

 heights; he here gives the method by which they are to be found, viz. As the 

 vapours will settle and rise where the air is of the same density with themselves; 

 it is only required to find the density of the air at any distance from the earth, 

 at several heights of the barometer ; which may be deduced from Dr. Halley's 

 two tables, Phil. Trans. N" 386, the first showing the altitude to given heights 

 of the mercury, and the second the heights of the mercury at given altitudes, 

 and knowing the degree of heat by the thermometer, because the density of 

 the vapour depends on the degree of heat of the season ; provided that proper 

 allowances be made for the great rarefaction of the air near the earth in hot 

 and dry weather, and the condensation of the vapours in their rise, by reason of 

 the air being colder at a little height above the earth, than just at its surface. 



An Account of some Observations relating to Natural History, made in a Journey 

 to the Peak in Derbyshire. By Mr. J. Martyn, F. R. S. N° 407, P- 22. 



The Peak in Derbyshire is famous for seven places, which our ancestors have 

 deemed wonders: 1. Chatsworth, a magnificent seat of his Grace the Duke of 

 Devonshiie; 2. Mam-tor; 3. Elden-hole; 4. The ebbing and flowing well; 

 5. Buxton well; 6. Peak's-hole; and 7. Pool's-hole. 



The first being a work, not of nature, but art, does not come within the 

 design of this account. Mam-tor is a huge precipice facing the east, or south- 

 east, which is said to be perpetually shivering, and throwing down great stones 

 u u 2 



