36'2 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



may be from some other particle of matter in motion acting on it, and commu- 

 nicating part of its motion to it, while it communicates an equal quantity of its 

 rest to the matter so acting on it, so that the quantity of motion and rest shall 

 be the same after the impact, in both bodies, as they were before: or, in other 

 words, a simple particle of matter in motion would always continue in motion, 

 in the same direction, if it did not meet with another, on which it acted; and 

 after the impact, there would be the same quantity of motion and rest in both 

 bodies taken together, as was before. If we consider equal motion, in direct 

 contrary direction, as rest; motion, or rest, produced in a body by the above 

 means, I shall call communicated. 



If 2 simple particles of matter, of any species, not farther distant from one 

 another than the sun is from the earth, were both at perfect rest, these 2 par- 

 ticles would instantly begin to move toward each other, if no other particle of 

 matter whatever existed. There would therefore be an impulse, producing mo- 

 tion between these bodies, without any contact. Motions produced in this way, 

 I call original motions. The first consideration with regard to any particular 

 motion is therefore, whether it be an original or communicated motion. If it 

 be an original motion, it will follow the laws of that particular species of original 

 motion; if it be a communicated motion, it will follow the laws of communica- 

 ted motion. Many observations shew, that muscular motion is not a commu- 

 nicated motion, and therefore an original one. 



In any system of bodies, or particles of matter, affecting one another only by 

 the motions already existing in them being communicated to each other, they 

 may diminish their motion, or bring one another to rest; but they never can in- 

 crease the motion existing in the whole. It happens frequently, that the motions 

 in the animal body are increased, without any alteration of external applications 

 to it; the cases are so numerous, that it is hardly worth bringing an example: 

 we might mention the increase at times of the circulation, and all the motions 

 of the fluids without the least new motion in the surrounding bodies, or inter- 

 ference, or even knowledge of the mind. This motion must therefore be ori- 

 ginal, and not communicated. 



In communicated motion, if one body be at rest, and a motion be communi- 

 cated to it by another, the power of the whole motion shall not be greater 

 than that in the communicating body at the time of the communication. If I 

 take out the heart of an animal, and cut off the auricles, it will in many cases 

 continue to contract and dilate for some time. If it be left to come to rest, and 

 if soon after a needle be introduced into the ventricle, placed transversely, and 

 if the interior surface of the ventricle be pricked gently by the needle, the vent- 

 ricle will contract with such power as to force the needle deep into it: in this 

 case, the force of the contraction of the ventricle is much greater than the 



