8^6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1783. 



nious work on the Temperature of different Latitudes ; viz. that in winter, when 

 the mercury in the barometer descends, the cold increases. More accurate ob- 

 servations on this subject, when the air is stationary, or when the wind conti- 

 nues in the same quarter, might lead to the discovery of the quantity of heat 

 squeezed out of the air by a certain pressure. 



The devaporation of aerial moisture. — As heat appears to be the principal 

 cause of evaporation, as well as of solution, and of fluidity in general, the pri- 

 vation of heat may be esteemed the principal cause of devaporation : for though 

 the air may, by its own power of attraction, or by means of the electricity it 

 may contain, dissolve and suspend a portion of water, as water dissolves and 

 suspends a portion of salt ; yet, by the application of cold, these are respectively 

 precipitated ; and therefore heat may be assumed as the immediate cause of these 

 solutions. Add to this, that water boils in vacuo with less heat ; that is, it 

 evaporates in vacuo faster or easier than in the open air, and therefore the attrac- 

 tive power of the atmosphere does not seem necessary to evaporation. Now, 

 when the barometer sinks (from whatever cause not yet understood this may 

 happen) the lower stratum of air becomes expanded by its elasticity, being re- 

 leased from a part of the super-incumbent pressure, and in consequence of its 

 expansion robs the vapour which it contains of its heat ; whence that vapour 

 becomes condensed, and is precipitated in showers, as is visible in the receiver of 

 an air-pump above-mentioned. 



There are however 2 other curious circumstances belonging to the devapora- 

 tion of water, which have not been perhaps much attended to. First, that the 

 deduction of a small quantity of heat from a cloud or province of vapour, com- 

 pared with the quantity of heat which was necessary to raise that vapour from 

 water, will devaporate the whole. This circumstance is evident in the operation 

 of common steam-engines, in which a small jet of water, whose heat is often 

 above 48°, perpetually devaporates the steam raised by a comparatively very 

 great quantity of heat under the boiler. This difficult problem is explicable 

 from the principles before established : if a small part of a province of vapour 

 be suddenly condensed, a vacuity takes place, and the contiguous walls of 

 vapour expand themselves into this vacuity ; and thus a large area of vapour, 

 perhaps of many miles in circumference, becomes more or less expanded ; by 

 this expansion cold is produced, that is, its capacity of receiving heat is in- 

 creased ; and the whole is devaporated. 



This very circumstance exactly takes place in the famous steam-engine of 

 Messrs. Watt and Boulton ; which, from the happy combination of chemical 

 and mechanic power, may justly be esteemed the first machine of human inven- 

 tion. In this excellent machine, after the cylinder is filled with steam, a com- 

 munication is opened between this reservoir of steam and a small cell, which is 



