VOL. LXV.] PHILOSOPHICAL TRANSACTIONS. 705 



the end of their institution. As this bounty of his majesty had been originally 

 granted for an astronomical purpose, the Society thought they could not dispose 

 of it on any more important object, or in any manner more consistent with the 

 intentions of their royal patron and benefactor, than by expending it on this as- 

 tronomical experiment of the attraction of a mountain, as what could hardly fail 

 of throwing light on the principle of universal gravitation, and was likely to lead 

 to new discoveries concerning the constitution of this earth which we inhabit, 

 particularly with respect to the density of its internal parts. 



The experiment being thus resolved on, the next thing to be done was to fix 

 on a projjer person to carry it into execution. Numerous and interesting as 

 my literary engagements are at the Royal Observatory, I had no thoughts of 

 undertaking this care and labour myself, till the council of the r. s. were pleased 

 to do me the honour to think my assistance necessary to insure the success of so 

 important and delicate an experiment. Their thinking so was a sufficient motive 

 with me to encounter whatever difficulties and fatigues might attend operations 

 carried on in so inconvenient and inclement a situation. But it was requisite I 

 should also have his majesty's permission for absenting myself so long from my 

 duty at the Royal Observatory. This his majesty was graciously pleased to grant, 

 and to allow me to stay as long as I thought necessary, to complete my very im- 

 portant observations. Such were the motives for undertaking this experiment, 

 and the preparations made for putting it in execution. I am now to give an ac- 

 count of the operations themselves. 



The quantity of attraction of the hill, the grand point to be determined, is 

 measured by the deviation of the plumb-line from the perpendicular, occasioned 

 by the attraction of the hill, or by the angle contained between the actual per- 

 pendicular and that which would have obtained if the hill had been away. The 

 meridian zenith distances of fixed stars, near the zenith, taken with a zenith 

 sector, being of all observations hitherto devised capable of the greatest accu- 

 racy, ought by all means to be made use of on this occasion : and it is evident, 

 that the zenith instrument should be placed directly to the north or south of the 

 centre of the hill, or nearly so. In observations taken in this manner, the zenith 

 distances of the stars, or the apparent latitude of the station, will be found as 

 they are affected by the attraction of the hill. If then we could by any means 

 know what the zenith distances of the same stars, or what the latitude of the 

 place would have been, if the hill had been away, we should be able to decide on 

 the effect of attraction. This will be found, by repeating the observations of 

 the stars at the east or west end of the hill, where the attraction of the hill, 

 acting in the direction of the prime vertical, has no effect on the plumb-line in 

 the direction of the meridian, nor consequently on the apparent zenith distances 

 of the stars ; the differences of the zenith distances of the stars taken on the 



VOL. XIII. 4 X .;„i:,^ viU u ^j;_i.jU i;:^*;; jA. j: 



