PEEFACE. 



IN his address before the British Association for the Advance- 

 ment of Science last year, the President remarked that the new 

 views of the Correlation and Conservation of Forces constitute the 

 most important discovery of the present century. The remark is 

 prohably just, prolific as has been this period in grand scientific re- 

 sults. No one can glance through the current scientific publica- 

 tions without perceiving that these views are attracting the pro- 

 found attention of the most thoughtful minds. The lively con- 

 troversy that has been carried on for the last two or three years 

 respecting the share that different men of different countries have 

 had in their establishment, still further attests the estimate placed 

 upon them in the scientific world. 



But little, however, has been published in this country upon the 

 subject; no complete work, I believe, except the admirable volume 

 of Prof. Tyndall on " Heat as a Mode of Motion," in which the 

 new philosophy is adopted, and applied to the explanation of ther- 

 mal phenomena in a very clear and forcible manner. I have, there- 

 fore, thought it would be a useful service to the public to reissue 

 some of the ablest presentations of these views which have ap- 

 peared in Europe, in a compact and convenient form. The selec- 

 tion of these discussions has been determined by a desire to com- 

 bine clearness of exposition with authority of statement. In the 

 first of these respects the essays will speak for themselves ; in re- 

 gard to the last I may remark that all the authors quoted stand 

 high as founders of the new theory of forces. Although I am not 



