VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 1 Q3 



IX. Quantity of Rain which fell at Barrowby near Leeds. By George Lloyd, 



Esq., F. R. S. p. 71- 

 This table of rain contains the quantity fallen in each month of 4 successive 

 years, the sums of which for those years are, in 1778, 28 inches; in 1779, 



Wilson was led to suspect some fallacy in his present grounds ; and that, in reality, the aberration 

 of the axes of the eye and of the sector might not at all differ, and that the ray passed relatively 

 along both, when they lay in the same straight line. For, according to this, the image of the star 

 would appear in its true place in the field, and of course the above phenomena could not exist. 

 He, on further consideration, found his suspicion was just, by the detection of a most material 

 circumstance, soon to be explained, which from the beginning had entirely escaped him. By taking 

 in this circumstance, and tracing its consequences, the whole discussion, at once, received a new 

 form ; and he was enabled fully to demonstrate, by the arguments stated in the present paper, that 

 the aberration of the axis of the eye and that of the telescope must precisely agree, notwithstanding 

 the acceleration of the ray on entering the eye, as resulting from Newton's doctrine of refraction. 



Having arrived at this important conclusion, at first so little apprehended, it could not but occur 

 that the same theory would hold true whatever magnitude or deepness was imputed to the eye. 

 Still the aberration of its axis would precisely agree with that of Dr Bradley's sector, when the ray 

 from the star passed relatively over both. From this it followed immediately, as an identical propo- 

 sition, that a telescope of any length filled with water, or any dense clear fluid, between the object 

 glass and the wires at the focus, would shew the very same aberration with Dr. Bradley's sector, or 

 any other telescope, having air only within it. 



This bein<r demonstrable, according to the Newtonian doctrine of refraction, Mr. Wilson saw that 

 it was immediately applicable to the purpose he had originally in view : because such an agreement 

 between a water and an air telescope, if actually found by observation, would constitute a proof of 

 the acceleration of light in the dense medium, in the ratio assigned by Newton. The reader 

 will perceive that this is the very thing which Mr. Wilson illustrates and proves by his present 



paper. 



In endeavouring to trace the circumstances of the displacement of the image necessarily arising 

 from his former premises, it comes to be considered how far the eye beheld the illumined wires in 

 the field of Doctor Bradley's sector, in their true places, notwithstanding the motion of the earth in 

 its orbit. This opened a question entirely new, namely, whether a terrestrial object, once seen in 

 the axis of a water telescope, steadily and immutably pointed, could ever appear to depart from die 

 axis by any new lateral motion given to both, by the orbitual motion, or otherwise. In this inquiiy, 

 the same circumstances he had formerly detected clearly pointed out that such a terrestrial aberra- 

 tion was an impossible tiling, according to the Newtonian doctrine of refraction ; and that the ob- 

 ject, once seen in the axis of the water telescope when immutably pointed, would still continue to 

 be seen in the axis, notwithstanding the direction of the orbitual motion, relative to the axis, con- 

 stantly varying according to the time of the day. 



The illustration and proofs of the various points now detailed, Mr. Wilson had fully made out before 

 the end of 1772, as can be shown by original letters in his possession, especially from one gentleman,* 

 of die first eminence as an astronomer and mathematician, who with the greatest liberality and can- 

 dour honoured, and warmly encouraged him in these researches by his correspondence. It was not 

 till he had arrived at a full understanding of the subject he learned that the late excellent and emi- 

 nent geometer Abbe Boscovich had proposed a similar experiment to him, and with the same view, 

 but concerning which he had not then made any publication. Afterwprds Lalande, in the 4th vol. of 

 his Astronomy, published in 1781, pages 687, 688, gave an account of Boscovich's ideas from his own 



* Dr. Maskelyne. 



VOL. XV. C c 



