324 Biographical Account of [May, 



tions which he had observed in the stars, and to which he gave 

 the name of the aberration of the fixed stars. Let us now 

 endeavour to communicate to the reader an idea of his expla- 

 nation. 



Let us conceive piles of small bodies moving in directions 

 parallel to each other ; as, for example, a rain without any wind, 

 falling down perpendicularly. Let us expose to this rain an 

 immoveable tube placed in the same vertical position. It is 

 obvious that the drop which enters at the upper orifice of the 

 tube will pass out at the other end without touching the inner 

 walls of the tube. 



But if we make the tube move parallel to itself, though its 

 position always remains parallel to the direction of the drops of 

 rain, it is obvious that the motion of the tube will cause the 

 drops to strike against one of its sides, and that the sooner, 

 according as the motion of the drops is slower, cpmpared with 

 that of the tube. And it is easy to demonstrate that if the 

 motion of both be equal, the drop of rain which falls upon the 

 centre of the upper opening of the tube will strike the inside wall 

 after having traversed exactly half the semidiameter of the tube ; 

 and that its direction in consequence will make an angle of 45^ 

 with the axis of the tube. Hence it follows that if we wish the 

 drops of water not to touch the tube notwithstanding its motion, 

 we must incline it 45° in the du'ection of its motion. If this 

 motion were to take place in the circumference of a circle, the 

 tube would describe round the vertical hne passing through the 

 centre of its base, a cone, the angle of which will be 90°. 



What has been said is meant to show that the inchnation of 

 the tube in order to allow the drops of rain, notwithstanding the 

 motion, to pass through the tube without striking againstits sides, 

 depends entirely upon the proportion between the velocity of its 

 motion and that of the drops of rain. The greater the velocity 

 of these drops compared with that of the tube, the less will it be 

 necessary to incline the tube. Hence if the velocity of the drops 

 were infinite compared with that of the tube, it would not be 

 necessary to inclme the tube at all ; because the drop would 

 reach the bottom of the tube the very instant that it entered its 

 top, and the tube, during such a space of time, could advance 

 only an infinitely small quantity. 



tVhen we apply this theory to the aberration of the stars it is 

 easy to see that the lines traversed by the drops of rain are the 

 rays proceeding from the stars ; that the tube which we have 

 supposed at first at rest, and afterwards in motion, is that of the 

 telescope, which serves to determine the position of the stars, 

 and which is always carried away by the motion of the earth 

 round the sun; and finally, that the velocity of hght being finite, 

 when compared with that of the earth in its orbit, the telescope 

 must change its position in proportion as this motion changes 

 its direction. Hence it follows that each star must have a series 



