i 
parts of the apparatus will be found in their initial state. 
A second evacuation, or, if we like it better, a second 
annihilation of the internal air, will again make the 
piston fall, and so on. 
The true moving power in this arrangement would be 
the weight of the atmosphere. Let us hasten to unde- 
ceive those who would think that they found in the facility 
with which we walk, and even run through the air, an 
index of the weakness of this motive power. With a 
cylinder of two metres in diameter, the effort made by 
the piston of the pump in descending, the weight that it 
could raise to an equal height with the cylinder at each 
of these oscillations, would be 35,000 kilograms. This 
enormous power, frequently renewed, may be obtained 
by a very simple apparatus, if we discover a prompt 
and economical method of alternately generating and 
destroying the atmospheric pressure at will, in a metal 
cylinder. 
This problem was solved by Papin. Its beautiful and 
great solution consists in substituting an atmosphere of 
steam for the common atmosphere; by replacing the 
latter with a gas which at 100 centigrade degrees has 
exactly the same elastic force, but with the important 
advantage, not possessed by the atmosphere, that the 
power of aqueous gas is very soon destroyed on its 
temperature being lowered, that it ends by almost en- 
tirely disappearing if sufficiently cooled. I should equally 
well characterize Papin’s discovery, and in few words, if 
I said, that he proposed to use the steam of water to make 
a vacuum in large spaces; and that this is besides a prompt 
and economical method.* 
382 JAMES WATT. 
* An English engineer, deceived no doubt by some imperfect trans- 
lation, asserted not long since that the idea of employing the steam of 
