402 GROVE'S CORRELATION OF PHYSICAL FORCES. 



the different sciences on each other, a knowledge of the elements of one being 

 essential to the successful prosecution of another. Thus physical astronomy 

 requires a knowledge of dynamics, and the practical astronomer must learn a 

 certain amount of optics in order to understand atmospheric refraction and the 

 adjustment of telescopes. The sciences are also shewn to have a common method, 

 namely, mathematical analysis ; so that analytical methods invented for the in- 

 vestigation of one science are often useful in another. 



The unity shadowed forth in Mrs Somerville's book is therefore a unity of 

 the method of science, not a unity of the processes of nature. 



Sir W. Grove's essay may be fairly called a popular book, as it has reached 

 its sixth edition. It is, therefore, not merely a record of the speculations nt 

 the author, but an index of the state of scientific thought among a large number 

 of readers. It has not the universal facility and occasional felicity of exposition 

 which distinguish Mrs Somerville's writings. No one could use it as a text- 

 book of any science, or even as an aid to the cultivation of the art of scientific 

 conversation. The design of the book is to shew that of the various forms of 

 energy existing in nature, any one may be transformed into any other, the one 

 form appearing as the other disappears. This is what is meant in the essay 

 by the "correlation of the physical forces," and the whole essay is an exposition 

 of this fact, each of the physical forces in turn being taken as the starting- 

 point, and employed as the source of all the others. 



We are sorry that we are not at present able to refer to the early reviews 

 of the essay as indicating the reception given to the doctrine by the literary 

 and scientific public at the time of its original publication. It has certainly 

 exercised a very considerable effect in moulding the mass of what is called 

 scientific opinion, that is to say, the influence which determines what a scientific 

 man shall say when he has to make a statement about a science which he does 

 not understand. Many things in the essay which were then considered contrary 

 to scientific opinion, and were therefore objected to, have since then become 

 themselves part of scientific opinion, so that the objections now appear unin- 

 telligible to the rising generation of the scientific public. 



Helmholtz's essay "On the Conservation of Force," published in 1847, un- 

 doubtedly masters a far greater step in science, but the immediate influence 

 was confined to a small number of trained men of science, and it had little 

 direct effect on the public mind. 



The various papers of Mayer contain matter calculated to awaken an interest 





