92 
MOVING FORCE. 
I 
Ca-oes of diffi- of niatte-r, and that they are the only ij( 
niltv in the which the science of mechanics borrows from experience. x| 
doctrines or ' 
moviii" force. It oiay be proper to observe here, that M. Laplace adopts p 
as first principles, only the two first of Sir Isaac Newton’s laws li 
of motion. , j 
It is surprising, that so many different opinions on this sub- |l 
ject should still be held, and it is not easy to understand how | 
so many good reasoners have, from the same data, drawn con- 1 
elusions so much at variance with each other. 
Fifty years ago, M. D’Alembert, speaking of the science of 
mechanics, observed, that “ En g6neral, on a etc plus occupc 
jusqua present i augmenter I’edifice qu’& en eclairer I'entree ; 
et on a pensc principalement ^ I'clever, sans donner a ses fonde- 
mens toute la soHdite convenable*.” 
No one will deny that, during the last fifty years, greavt ad- 
vances have been/nade in the application of mechanical prin- 
cipies iC ihs investigation of the motions of the heavenly Iw- 1 
dies. But as far as these principles have been adapted to prac- | 
tical uses, may not M. D’Alembert’s observation be, with some | 
justice, applied to the present state of mechanical science ? or, I 
may it not be said that, not only the entrance, but the interior I 
of the structure is not very conveniently arranged for the ofcu- I 
pations of life. I 
But there is anotlier observation of M. D’Alembert, whicih I 
has, on the present occasion, still stronger claims on my atten- 
tion. He says, “ mais il semble que la plfipart de ceux qui ont 
traite la question de la mesure des forces, ayent craint de la 
trailer en peu de mots.’’ 
Although the censure be severe, it may be just, and I shall 
endeavour to profit by it. Some repetitions, however, in dis- 
cussions of this kind are unavoidable. 
In the observations which I have made, as well as in those 
which I have yet to make, on various passages in some of the 
best authors on mechanics, I hope to escape the charge of be- 
ing, in any degree, disrespectful towards them. I am sensible, 
* Trait4 Dyuaniique, Discours prelim, p. 4. ' 
that 
