382 JAMES WATT. 



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.* 



* An English engineer, deceived no doubt by some imperfect trans- 

 lation, asserted not long since tbat the idea of employing the steam of 



