230 Mr. C. W. Siemens, on a [April U, 



had descended, the rotation of the wheel would continue uniformly, 

 but it might soon be brought to a stop by immersing it in a basin 

 filled with water. In this case the water was the recipient of the 

 force due to the falling weight residing in the wheel ; and by 

 repeating the same experiment a sufficient number of times, we 

 should find an increase of temperature in the water, a fact discovered 

 by Joule, in 1843, which first proved the identity of heat and 

 dynamic effect, and established their numerical relation. If the 

 weight falling over a pulley were one pound, and the distance 

 through which it fell one foot, then each impulse given to the 

 wheel would represent one foot pound, or commonly adopted unit 

 of force ; and if the water contained in the basin weighed also one 

 pound, it would require 770 repetitions of the experiment of arrest- 

 ing the wheel in the water, before the temperature of that water 

 was increased by one degree Fahrenheit. 



Another illustration made use of, was that of a hammer falling 

 in vacuo upon a perfectly elastic anvil. The hammer would, under 

 these circumstances, rebound to precisely its original elevation ; and 

 granting the perfect elasticity of both hammer and anvil, neither 

 sound nor heat would be produced at the point of concussion. 

 If a piece of copper v/ere suddenly introduced between anvil and 

 hammer, the latter would not rebound, but would make the copper 

 the recipient of the expended force. If the hammer were now lifted 

 again and again by an engine, and the piece of copper were turned 

 about on the anvil, so that at the end of the operation it had pre- 

 cisely the same form as at the commencement, then no outward effect 

 would be produced by the force expended, but the piece of copper 

 would be heated perhaps to redness ; and if the engine employed 

 to lift the hammer were perfect, then the heat produced within the 

 copi>er should be sufficient to sustain its motion. 



A familiar instrument for converting force into heat was the 

 fire-syringe. The force expended in compressing the air imparted 

 a sufficient temperature to the same to ignite a piece of Gerraaa 

 tinder (about 600^ Fah.). When the plunger of the syringe was 

 drawn back, it might be observed that the temperature of the 

 enclosed air was again reduced to its original degree, because the 

 heat developed in compression of the air had been spent again in 

 its expansion behind the piston. If the expansion of the heated 

 and compressed air had been without resistance, no reduction of its 

 temperature could have taken place, because no force would be 

 obtained ; a fact which had been recently proved by Kegnault, and 

 which was perhaps the strongest proof in favour of the dynamic 

 theory of heat that could be brought forward. If the heated and com- 

 pressed air in the fire-syringe could be produced by some external 

 cause and be introduced behind the plunger after it had descended 

 freely to the bottom, then the force imparted to the plunger in the 

 expansion might be turned to some useful purpose, and a dynamically 

 perfect engine might be obtained. But although the elevated tem- 



