426 FKAGMENTS OF SCIENCE. 



a mechanical power, prepared for action, as a watch- 

 spring is when wound up. Suppose, for the sake of 

 illustration, that 8 Ibs. of oxygen and 1 Ib. of hydrogen 

 were presented to one another in the gaseous state, and 

 then exploded; the heat evolved would be about 1 

 Fahr. in 60,000 Ibs. of water, indicating a mechanical 

 force, expended in the combination, equal to a weight 

 of about 50,000,000 Ibs. raised to the height of one 

 foot. Now if the oxygen and hydrogen could be pre- 

 sented to each other in a liquid state, the heat of com- 

 bination would be less than before, because the atoms 

 in combining would fall through less space.' No 

 words of mine are needed to point out the com- 

 manding grasp of molecular physics, in their relation 

 to the mechanical theory of heat, implied by this state- 

 ment. 



Perfectly assured of the importance of the principle 

 which his experiments aimed at establishing, Mr. Joule 

 did not rest content with results presenting such dis- 

 crepancies as those above referred to. He resorted in 

 1844 to entirely new methods, and made elaborate ex- 

 periments on the thermal changes produced in air dur- 

 ing its expansion: firstly, against a pressure, and there- 

 fore performing work; secondly, against no pressure, 

 and therefore performing no work. He thus estab- 

 lished anew the relation between the heat consumed and 

 the work done. From five different series of experi- 

 ments he deduced five different mechanical equivalents; 

 the agreement between them being far greater than 

 that attained in his first experiments. The mean of 

 them was 802 foot-pounds. From experiments with 

 water agitated by a paddle-wheel, he deduced, in 

 1845, an equivalent of 890 foot-pounds. In 1847 he 

 again operated upon water and sperm-oil, agitated 

 them by a paddle-wheel, determined their elevation of 



