fiO PHILOSOPHICAL TEANSACTIONS. [aNNO 1770. 



day;" whence, on the 28lh, at Q^, it was 1"' 51^5 slower than mean time. The 

 equation of time on the 28th at Q^ was 2*" 47^5, whence the immersion was at 

 Q^ 45™ 44' apparent time. 



The telescope made use of was one of DoUond's, with a triple object glass of 

 33J- inches focal distance, and Which magnifies 52 times. 

 XXX. On the Transit of Fenus. By John Winthrop, Esq., F. R. S., at Cam- , 



bridge, Neiv England, p. 358. , W 



I find, says Mr. W. that Mr. Bliss and Mr. Hornsbj^, in their calculations in 

 the Philos. Trans., suppose the phases of the transit of Venus, to be accelerated 

 by the equation for the aberration of light, which amounts to 55* of time. 

 According to my idea of aberration, I should think the transit would be 

 retarded by it, I can very easily suppose that I am in an error; and that I may 

 more readily be led out of it, I beg leave to lay before you the several steps by 

 which I have been led into it. And I think, it will be best to take some similar 

 instance, rather than to consider the thing in a general abstract manner. 



1. Let the parallelogram e represent a vessel sailing in the i^ j^ if 

 line LR, from left hand to right , and s, a fixed station, e. g. '■ 

 a castle, discharging balls in the right line sm, perpendicular ; 

 to the route of the vessel. If the vessel had been at rest, a 

 ball arriving at the middle of it, m, would have gone right 

 across it, to n. But as it is supposed to be sailing, the ball 

 will not go right over from m to n, but will cross the deck 

 obliquely, in another right line, as mo, and so will be left 

 behind towards the stem as much as the vessel had gone 

 forward, while the ball was crossing it; and mn will be to no 

 as the velocity of the ball to the velocity of the vessel. Thus, . r-^ '■ — i „ 

 to the people on board, the Ijall would seem to move ^J^ 

 obliquely across the deck, as if it came from some point x in the line 

 CM, produced, instead of coming from s. And a tube capable of receiving the 

 ball, would allow the ball to pass through it without striking its sides, if it were 

 inclined forward in the direction om; which it would not do in any other 

 situation. The angle omn or smt answers to the aberration ; and supposing s 

 to be the sun, and e, the earth, this angle is 20" ; and the general effect is, to 

 make the sun, or any fixed star, to appear further that way towards which the 

 earth is moving. 



2. Let us suppose another vessel v, between s and e, sailing the same way as 

 E, in a parallel direction. If both the vessels sailed with the same velocity, a ball 

 from V coming to m, would go right across to n, just as if both of them had 

 been at rest; because the ball, while crossing the vessel e, would be carried just 

 as far to the right hand as the points m and n are. And a tube to receive it 



B3 



