VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 36^ 



communicated from one body to another. Des Cartes is the first we know ot 

 who gave any attention to the subject; and the result of his inquiries is what 

 might reasonably be expected from so whimsical and romantic a genius; he 

 blundered in this, as in all other cases, where he was not confined to pure 

 mathematical reasonings. Our countryman. Dr. Wallis, made a real progress 

 in this science, by discovering that fundamental law in the communication of 

 motion, viz. that action is equal to re-action, and always in contrary directions: 

 Wren, Huygens, confirmed the same thing; and the whole theory of the 

 collision of bodies, and their mutual actions on one another, seemed to be 

 advancing fast towards perfection. But a new opinion was now started by 

 M. Leibnitz concerning tiie forces of bodies in motion. The force of a body in 

 motion and its momentum had hitherto been considered as synonymous terms, 

 and had alike been measured by the quantity of matter and velocity conjointly. 

 On the contrary, Leibnitz and his followers afiirmed, that the force was propor- 

 tional to the quantity of matter in the moving body and the square of its velo- 

 city. It is needless to relate all that passed on both sides : so material an opposi- 

 tion in sentiment necessarily produced very warm contention; and, as it gene- 

 rally happens in other disputes, we do not hear of any conviction being pro- 

 duced on either side. 



After surveying tiie arguments of the disputants, it is not easy to say, whe- 

 ther the agitation of the question before us has contributed to retard or advance 

 the progress of truth and science. On the one hand, many ingenious experi- 

 ments have been made, many curious problems invented and resolved, which 

 probably would never once have been thought of by men who were in the 

 pursuit of truth in a more cool and deliberate way: and, on the otlier hand, it 

 may justly be affirmed, that the violence of prejudice and party-spirit has so mucli 

 clouded the reasonings of the best writers, that we sensibly feel their influence 

 to this day. I need not dissemble: it is a serious persuasion, that the laws by 

 which motion is communicated are still very materially mistaken by sensible 

 persons, that induced me to throw together the following hints, and to lay them 

 before the r>. s. The right understanding of these laws is of the last importance 

 in practice: the good or bad success of some very expensive projects has depended 

 on it; and certain excellent artists have been disappointed in the execution of 

 their plans, and unable to reconcile the apparent contradiction between theory 

 and experiment. From the length of time which has elapsed since Leibnitz 

 first advanced his new opinions, and the abilities of philosophers who engaged in 

 the contest, one might have expected, that the whole matter would long before 

 this have been cleared up in a satisfactory manner; especially when we consider, 

 that the communication of motion from one bod) to another is what every 

 moment happens before our eyes, and that particular experiments are made in 



VOL. XIV. 3 B 



