648 PHILOSOPHICAL TRANSACTIONS. [aNNO I762. 



a mean of 52 observations, taken at the more regular rising and fallings of the 

 water, he found the altitude to be 3-^. He always looked at his watch before 

 he began to note the height of the water, and looked at it again when he had 

 finished the experiment ; the medium of the two times he set down as the true 

 time of the observation. The times set down are exact to the minute. 



Mr. M. says nothing with respect to any conclusions that may be drawn from 

 the above observations, except that the greatest rise and fall of the water, ob- 

 served at the sysigies of the sun and moon, is about 1 3 divisions of the post, or 

 39 inches ; and that the smallest rise and fall in the quadratures, is somewhat 

 less than 7 divisions of the post, or about 20 inches; and that the mean time of 

 high water happens 2^ IS'" after the moon's passing the meridian, though in the 

 course of every fortnight the said interval is very much varied by the different 

 influence of the sun at different times, as the theory requires. 



XCIX. Extract of a Letter from M. de la Lande, at Paris, to the Rev. Nevil 

 Maskelyne, F. U.S. Dated Paris, Nov. 18, 17 62. p. 607. 



I am glad that you have proved, from your own experience, the exactness of 

 the observations of the distance of the moon from stars for finding the longitude 

 at sea, as M. de la Caille had done in 1753. I am as fully convinced as you can 

 be of these advantages, and am not a little pleased to learn that you are about 

 printing a concise method of computing the corrections of refraction and parallax. 



In the sector, which our members of the academy carried with them to V y/- 

 the north, the plumb-line descends from an angle as in this figure, so that it (\\ 

 is obliged to fall into the vertex of the angle; and it passes over a point 

 A, with which it is made to correspond, by the help of a microscope. It 

 is a pity that Mr. Sisson neglected so essential a circumstance in your 

 sector, but that is not your fault. The sector, with which M. de la Caille ^^ 

 made all his observations, and which is come into my hands since his death, has 

 a fine needle at the centre, from which the silver wire is suspended by a [1 

 loop, thus 



M. Pingre, who is returned from the island of Rodrigues, has found the 

 parallax of the sun to be the same as I have done; namely Q"±. I am not 

 surprized that you find it to be only 8*4-, since the Swedish observations, which 

 appear to me to be very good, make it still less than you have found it. These 

 uncertainties arise from our not having the difference of the meridians of the 

 Cape, Rodrigues, Tobolski, Paris, and London, well determined. You are 

 therefore quite right to collect together the observations of Jupiter's satellites, 

 which will serve to find these longitudes. I thank you for those which you have 

 sent me, and I have hereto added those of the first satellite which were made at 

 Paris in 1761, for one can scarcely employ any but these for this purpose. 



