07h Artificial Dryings <^c. Iry tfie Air-Pump. 57 
paper of very great interest, and, in my humble opinion, con- 
tdinsj not only the method of artificial desiccation, but also the 
principles and almost the whole process of artificial congelation 
by means of evaporation under ^n exhausted receiver. 
Having been perplexed by the disagreement of the pear gauge 
and the common barometrical gauges, in their indications of the 
degree of exhaustion in the receiver, Mr Nairne was induced to 
undertake a series of experiments, in order to investigate the 
cause of the disagreement. He exhibited to the Honourable 
Mr Cavendish, Mr Smeaton, and other members of the Royal 
Society of London, an experiment in which this disagreement 
amounted to some thousand times, and Mr Cavendish immedi- 
ately furnished him’ with a satisfactory explanation of the fact. 
“ It appeared,” he said, “ from some experiments of his father 
Lord Charles Cavendish, that water, whenever the pressure of 
the atmosphere on it is diminished to a certain degree^ is imme- 
diately turned into vapour, and is as immediately turned back 
again into water on restoring the pressure. This degree of pres^- 
sure is different, according to the heat of the water. When the 
heat is 72° of Fahrenheit’s scale, it turns into vapour as soon as 
the pressure is no greater than that of three quarters of an inch of 
quicksilver, or about one-fortieth of the usual pressure of the at- 
mosphere ; but when the heat is only 41°, the pressure must be 
reduced to that of a quarter of an inch of quicksilver before the 
water turns into vapour. Hence it follows, that when the re- 
ceiver is exhausted to the above mentioned degree, the moisture 
adhering to the different parts of the machine will turn into 
vapour, and supply the place of the air which is continually 
drawn away by the working of the pump, so that the fluid in 
the pear gauge, as well as that in the receiver, will consist in good 
measure of vapour. Now, letting the air into the receiver, all 
the vapour within the pear gauge will be reduced to water, and 
only the real air will remain uncondensed; consequently the 
pear gauge shews only how much real air is left in the receiver, 
and not how much the pressure or spring of the included fluid is 
diminished, and that equally, whether it consist of kif or vapour.” 
Now, this ingenious explanation, which Mr Nairne considered 
as perfectly satisfactory, contains the fundamental principle of pro- 
ducing artificial dryness and cold in the receiver of an air-pump, 
