VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 377 



kept cold by surrounding water, and free from air by an air-syringe adapted to 

 it. What then happens? The corner of the steam in the cylinder next to this 

 vacuum, with which it now communicates, rushes into it, and the whole steam 

 in the cylinder is thus suddenly expanded, and instantly devaporated: whence 

 the very quick reciprocations of the piston; and that, though the cylinder itself 

 is always kept as hot as boiling water, that is, as hot as the steam was previous 

 to its devaporation. 



Conclusion. — 1. When a small portion of air, suppose a few acres, becomes 

 suddenly contracted into a less compass, either by incidental cold, or by any 

 other cause not yet understood (as the combination of dephlogistic and inflam- 

 mable gases,) the air next in vicinity suddenly expands itself to occupy the 

 vacuity ; and by its expansion produces cold and devaporates, and then becomes 

 compressible into less space than it occupied before it parted with its vapour. 

 This then gives occasion to the next circum- ambient portion of air to go through 

 the same process, that is, to expand, attract the heat from its vapours, devapo- 

 rate, and then become compressible into less space ; and thus, from a small and 

 partial contraction or dimunition of air, it seems possible to devaporate a great 

 province. 



2. The vapour of a great province of air being thus condensed, would leave 

 a great vacuity in that part of the atmosphere, which would be supplied by winds 

 rushing in on all sides. Suppose this to happen to the north of our climate, a 

 south-west wind would be produced here, which is otherwise very difficult to un- 

 derstand: and if it should ever be in the power of human ingenuity to govern 

 the course of the winds, which probably depends on some very small causes; by 

 always keeping the under currents of air from the s. w. and the upper currents 

 from the n. e., I suppose the produce and comfort of this part of the world 

 would be doubled at least to its inhabitants, and the discovery would thence be 

 of greater utility than any that has yet occurred in the annals of mankind. 



V. Some Observations on the Heat of Wells and Springs in the Island of Ja- 

 maica, and on the Temperature of the Earth below the Surface in Different 

 Climates. By John Hunter, M.D., F.R.S. p. 53. 



The great difference, says Dr. H. between the temperature of the open air, 

 and that of deep caverns or mines, has long been taken notice of, both as matter 

 of curiosity and surprize. After thermometers were brought to a tolerable de- 

 gree of perfection, and meteorological registers were kept with accuracy, it 

 became a problem, to determine what was the cause of this difference between 

 the heat of the air and the heat of the earth; for it was soon found that the 

 temperature of mines and caverns did not depend on any thing peculiar to them ; 

 but that a certain depth under ground, whether in a cave, a mine, or a well, 

 VOL. XVI. 3 C 



