1818.] Analyses of Books. ■ 445 



principle requires to be almost formed anew. You will probably 

 recollect a paper " On the Measure of Moving Force," by Mr, 

 Ewart in the second volume (second series) of the Memoirs of 

 the Manchester Society, which was read Nov. 18, 1808. This 

 paper was reprinted by Mr. Nicholson in the 36th vol. of his 

 journal, and it was also reprinted about the same time in the 

 Repertory of Arts. You noticed it yourself in the Annals of 

 Philosophy, vol. i. p. 462, and again briefly in your review of 

 the progress of science during the year 1813, in vol. hi. of the 

 Annuls, p. 8. Excepting these, I do not know that any other 

 public notice has been taken of this paper. 



I fully accord with M. Petit that our elementary treatises on 

 mechanics are extremely defective in developing the principles 

 of moving force, and in their application to explain the action of 

 machines. The object of this letter is to recommend to 

 M. Petit, and to others who may be interested in the subject, to 

 peruse the above-mentioned essay. Mr. Ewart has pointed out 

 the source of numerous errors and inconsistencies in some of the 

 best writers on mechanics, and has, I think, succeeded in giving 

 satisfactory explanations of various cases (especially those in 

 which moving force is expended in producing change of figure) 

 which were before involved in much obscurity. 



In estimating the power communicated by a stream of water 

 to an under-shot water-wheel, M. Petit gives the same result 

 which Mr. Ewart has given (p. 231), when supposing it to act in 

 a similar manner ; and they agree also in their statement of the 

 maximum effect of the reaction of water in the machine known 

 by the name of Barker's Mill, supposing the waters to issue with 

 the velocity due to the pressure. If M. Petit has not seen Mr. 

 Ewart's paper, their agreement in these results is the more 

 remarkable ; as I believe they are different from those obtained 

 by all other writers who have attempted solutions of the same 

 cases. The investigation of both these cases, however, appears 

 to have been pursued considerably further by Mr. Ewart than 

 by M. Petit : and he has corroborated his conclusions by some 

 original and ably conducted experiments. 



I remain, yours truly, 



John Dalt-on. 



Article VII. 



Analyses of Books. 



Philosophical Transactions of (he Royal Society of London 

 for the Year 1818. Part 1. 



This part contains the following papers : 

 I. On the great Strength ghen to Ships of War by the Appli- 

 cation of diagonal Braces. By Robert Seppings, Esq. F.R.S. 



