Sfxt. IV.] Motion and Moving Forces. 41 



possesses for iron, decomposes part of tlie fluid m 

 the iron, and each of its ingredients occupies oj)po- 

 site ends of the bar. Bars in this state approach, 

 if the ends nearest each other contain dilTercnt in- 

 gredients, but recede if they contain the same*. 



SECTION IV. 



MOTION AND MOVING FORCES. 



This part of science also, within the century 

 under consideration, has received no small im- 

 provement. The laws of motion^ as laid down by 

 sir Isaac Newton, though found, hy succeeding 

 philosophers, to be in general correct, were yet by 

 no means perfectly so. His princi})les of motion 

 in resisting mediums particularly failed, when 

 brought to the test of accurate experiment. Nu- 

 merous have been the attempts to supply the de- 

 fects, and to correct the errors of these principles : 

 among which the labours of D. Bernoulli, and of 

 M. d'Alembert, deserve to be considered as by 

 far the most distinguished and successful. The 

 latter in particular, in the course of his investiga- 

 tions, discovered a general rule, adequate to the 

 determination of many important questions in the 

 science of motion, and applying to the most com- 

 pound and perplexing cases f. 



The inaccuracy of Newton's principles, with 

 regard to projectiles, was first ascertained and an- 

 nounced by M. Ressons, a French artillerist, in 

 I7I6. Nothing material, however, was. done to- 



* See Essai sur rOn'gine dcs Forces Magniti(^iiC6; l/SS. 

 t Condorcct on the Mind, p. 2/5. 



