392 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



upper face of the piston with, a force proportional to the surface, and 

 having a definite relation to the degree of exhaustion obtained. 



A humble French physicist, Denis Papin, whom the revocation of 

 the Edict of .Nantes forced into exile, afterwards 1 tried to improve 

 upon the machine proposed by Huygens ; a machine which, moreover, 

 in the opinion of its inventor, " could be used not only to raise all 

 kinds of heavy weights, and water for fountains, but also project 

 bullets and arrows with considerable force, like the balista of the 

 ancients." But shortly after, in 1690, he proposed to substi- 

 tute for gunpowder another agent, which, like it, could produce a 

 vacuum beneath the piston, and leave it exposed in this way to the 

 whole pressure of the atmosphere. 



This agent was steam, with which Papin was already familiar, since 

 in 1681 he had invented his celebrated boiler, or new digester, of 

 which we shall speak hereafter. We now give briefly a description 

 of the first steam-engine as it was conceived by Papin, and the 

 explanation of its effects which is easily intelligible. 



In Fig. 2*72 B is a piston provided with a vertical rod, D, and 

 movable in a cylinder of the same diameter, into the inside of which 

 is introduced- a little water. In the piston is bored a hole which 

 can be closed at pleasure by the rod M. 



Let us suppose the piston placed in the cylinder just in contact 

 with the water (which has passed through the opening, which is then 

 closed by means of the rod). Let us now place the cylinder, which is 

 made of metal, upon a hot fire. The water is soon reduced to steam, and 

 this, by its elastic force, overcomes the weight of the piston and the 

 pressure of the atmosphere, and drives the piston to the top of the 

 cylinder ; when the piston arrives at the end of its stroke, a narrow 

 rod, c, movable about one of its ends, and until now kept in contact 

 with the piston-rod by the spring, G, enters an opening in the rod, 

 as soon as that opening is brought opposite its extremity by the 

 ascent of the piston. At this moment then the motion is stopped. 

 We now take away the fire from beneath the cylinder, and it and the 

 water contained in it becomes cool, the vapour condenses, and a 

 vacuum is produced below the piston, so that if the rod be taken out 

 of the opening in it, it will be pressed down by the weight of the 



1 The first attempt dates from the year 1688. 



