712 PHILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



for setting the instrument by the sun, I should have lost 4 good days observa- 

 tions, which were -§- of those I took on this side of the hill, with the plane of 

 the instrument turned to the west, and been retarded near 3 weeks in my 

 observations ; and, as the opportunities of weather fit for observing at all were 

 but very rare, I might have been thrown back into the winter, and defeated of 

 making so complete a set of observations on the north side of the hill as I had 

 got on the south side, whose correspondence would thus have been rendered less 

 perfect. I had the satisfaction, however, when I drew the meridian line on the 

 floor of the observatory, by the equal altitudes of the sun taken on the 15th, to 

 find it agree perfectly, even to the same minute, with the position of the instru- 

 ment, as determined by the transits of the stars. But no one will doubt of the 

 superior ease and readiness afforded by the latter method in preference to the 

 other. 



On the 20th of September I completed the observations with the plane of 

 the sector, turned to the west, having observed 32 stars, and taken 68 observa- 

 tions in all. On the 22d, I turned it about with the plane to face the east, and 

 set it again in the meridian, by putting it parallel to its former position, by means 

 of the meridian line secured by marks made on picquets let into the ground 

 perpendicularly below the plane of the instrument, before it was turned. 

 Between this time and the 24th of October, I observed 37 stars, and took lOO 

 observations in all, with the plane of the instrument facing the east : and thus I 

 completed my whole series of observations with the sector, having observed 43 

 different stars in all, on both sides of the hill, and taken 337 observations. 



As a few observations, taken with so excellent an instrument as this zenith 

 sector, would have been sufficient to determine the apparent difference of lati- 

 tude of the 2 stations of the observatory, to a second or 2 ; 1 am apprehensive 

 I may be thought by many to have multiplied observations unnecessarily. How- 

 ever that may be, I apprehend, that doubling the observations in each station 

 of the observatory, by taking them with the plane of the instrument alternately 

 facing the east and west, will be allowed to be a proper step, as the line of col- 

 limation of the instrument is thus separately determined at each station, and so 

 all danger of any alteration happening in the same, in its removal from one side 

 of the hill to the other, is entirely obviated. I had indeed all the reason in the 

 world to think, that the sector was carried from one station to the other without 

 the least accident : but still it was proper to guard against what was possible to 

 happen. 



But I had reasons also for multiplying the observations made in the same 

 position of the instrument. It was important to demonstrate the exactness of\ 

 the instrument from the near agreement of a number of observations taken with 

 it, as its excellence was not to be entirely presumed, unless this proof could be 



