258 PHILOSOPHICAL TRANSACTIONS. [aNNO l66S. 



5 defin. 6 Eucl.) the percussion at D, is to the percussion at F, in the ratio 

 compounded of the ratio of the sine of the angle of incidence A D E, to the 

 sine of the angle of incidence A F E, and of the ratio of the velocity in A D to 

 the velocity in A F ; which was to be demonstrated. 



It makes no difference that this demonstration is confined to equal motions in 

 right lines and resisting planes; for it is tnie in every case, since the percussions 

 are made in the point in which the right and curved lines coincide and agree : 

 but if the percussions be not made in points, from these no geometrical con- 

 siderations can be given, but the defect of the conclusion is to be judged of 

 according to the defect of matter from the requisite conditions ; as it ought 

 always to be, when geometrical demonstrations are applied to a physical body. 



In Angeli's reply to Manfredi, he mentions an experiment, which, as it was 

 related to him by a Swedish gentleman, had been made with all due circum- 

 spection by Descartes, to prove the motion of the earth. The experiment was ; 

 he caused a cannon to be erected pei*pendicular to the horizon ; which being 24 

 times discharged in that position, the ball fell 22 times towards the west, and 

 only twice towards the east.* 



/l?i Enlargement of the Observations formerly published in Numb. 27' 

 % Dr. Stubbes. N' 36, p. 699- 

 Of no consequence now to any one. 



Extract of a printed Letter, addressed to the Editor, by Dr. Denis, 

 of Paris, touching the Differences that have arisen about the Trans- 

 fusion of Blood. Dated Paris, May 15, 1668. N' 36, p. 710. 



You have sensibly obliged me in assuring me, by your letter of April 29, 

 that the magistrates of London had not at all concerned themselves to prohibit 

 the practice of the transfusion of blood, and that that operation had been hi- 



* There seems to be a strange blunder made here, both by Angeli and the Swedish gentleman, 

 with regard to Descartes's object in this experiment j since it rather proves the composition of forces, 

 than the earth's motion. For as it is constantly found that, in all such experiments as this, the ball 

 falls down again very nearly on the same spot from whence it was discharged, the inference would ra- 

 ther be (independent of the composition of forces) that the earth did not move. For, granting the 

 earth's motion, either diurnal or annual, or both, then the ball must fall veiy far indeed from the place 

 of the gun, viz, by more than tliree miles in the middle latitudes, in consequence of the diurnal mo- 

 tion, and not less then 300 miles by the annual motion, were it not for tlie composition of motion, or 

 of forces, supposing the ball to ascend only to the moderate height of 1000 feet. So that, such com- 

 position being a fact very well established, the experiment proves nothing as to the earth's motion, 

 neither for nor against it. But, granting such motion, then tlie circumstance of the ball falling down 

 qgain near the place of the cannon, is a splendid proof of tlie composition offerees or motion. 



