2 Mr. J. P. Joule on the (economical production of 



produce ; these, which may be termed thermo-dynamic engines, 

 include steam-engines, air-engines, &c. 



The process whereby muscular eflfort is developed in the living 

 machine is, as might be expected, involved in great obscurity. 

 Professor Magnus has endeavoured to prove that the oxygen 

 inspired by an animal does not immediately enter into combina- 

 tion with the blood, but is mechanically conveyed by it to the 

 capillary vessels within the muscles, where it combines with cer- 

 tain substances, converting them into carbonic acid and water. 

 The carbonic acid, instead of oxygen, is then absorbed by the 

 blood, and is discharged therefrom when it reaches the lungs. 

 Taking this view, we may admit with Liebig, that at each effort 

 of an animal a portion of muscular fibre unites with oxygen, and 

 that the whole force of combination is converted by some myste- 

 rious process into muscular power, without any waste in the form 

 of heat. This conclusion, which is confirmed by the experiments 

 related in a joint memoir by Dr. Scoresby and myself, shows 

 that the animal frame, though destined to fulfill so many other 

 ends, is as an engine more perfect in the oeconomy of vis viva 

 than any human contrivance. 



The electro-magnetic engine presents some features of simi- 

 larity to the living machine, and approaches it in the large pro- 

 portion of the chemical action which it is able to evolve as 

 mechanical force. If we denote the intensity of current elec- 

 tricity when the engine is at rest by a, and the intensity of cur- 

 rent when the engine is at work by i, the proportion of chemical 



force converted into motive force will be , and the quantity 



h ^ 



wasted in thCi form of heat will be -. Now from my own expe- 

 riments, I find that each grain of zinc consumed in a DanielPs 

 battery will raise the temperature of a lb. of water 0°-1886 ; and 

 that the heat which can increase the temperature of a pound of 

 water by one degree, is equal to the mechanical force which is 

 able to raise a weight of 772 lbs. to the height of one foot, or 

 according to the expression generally used, to 772 foot-pounds. 

 Therefore the work developed by a grain of zinc consumed in a 

 DanielFs battery is given by the equation 



^^ 145-6(<^-&) _ 

 a 



We now come to the third class of engines, or those in which 

 the chemical forces act through the intervention of heat. In the 

 most important of these the immediate agent is the elasticity of 

 vapour or permanently elastic fluids. In a very valuable paper 

 on the dynamical theory of heat. Professor William Thomson 



