VOL. LXV.] PHILOSOPHICAL TRANSACTIONS. 707 



Thus there were three principal operations requisite to be formed. 1 . To find 

 by celestial observations the apparent difference of latitude between the two sta- 

 tions, chosen on the north and south sides of the hill. 2. To find the distance 

 between the parallels of latitude. 3. To determine the figure and dimensions of 

 the hill. 



I arrived at the hill of Schehallien on the last day of June, and found the ob- 

 servatory and instruments there, which had been brought down some time before 

 from London to Perth on board a ship, and thence conveyed over land to the hill 

 under the care of Mr. Reuben Burrow, my late assistant at the Royal Observa- 

 tory. The observatory was fixed half-way up the south side of the hill, as the 

 place where the effect of the hill's attraction would be at the greatest, and it was 

 placed in the like manner when it was afterwards removed to the north side. A 

 circular wall was raised, 5 feet in diameter, and covered at top with a moveable 

 conical roof for sheltering the astronomical quadrant; and a square tent was set 

 up for receiving the transit instrument, all near the observatory. A bothie, or 

 temporary hut, was also made near it, for my residence, while attending the as- 

 tronomical observations on this side of the hill. I first put the sector, nearly in 

 the meridian, by means of the variation compass; but, through the badness of 

 the weather, which was almost continually cloudy or misty, I could not before 

 the middle of July get a sufficient number of observations with the astronomical 

 quadrant, to know the state of the clock, in order to draw a meridian line on the 

 floor of the observatory, for setting the sector truly in the plane of the meridian. 

 The first obsei-vations which I made with the sector, after it was set truly in the 

 meridian, were on the 'ZOth of July. Between this time and the end of the 

 month, I observed the zenith distances of 34 stars, some to the north and some 

 to the south of the zenith ; and many of them several times over, having takea 

 76 observations in all, with the plane of the sector turned to the east. On the 

 first of August I turned the plane of the instrument about, to face the west, 

 and set it in the meridian again, by means of the meridian line drawn on the 

 floor the 26th of July, and secured by piquets driven into the ground; and be- 

 tween that and the 15th of the same month, I observed 39 stars, including most 

 of those taken in the former position of the instrument, and took Q3 observa- 

 tions in all. • 



And here let me take notice of a method, which I fell upon, of verifying the 

 jxjsition of the sector, with respect to the plane of the meridian, which, had I 

 thought of it at first, would have saved me much trouble; and therefore I will 

 now mention it, as it may be useful to future observers. It consists in observing 

 the transits of two stars, differing considerably in declination from each other, 

 across the vertical wire of the sector, and comparing the observed difference of 

 their transits with the known difference of their right ascensions. If they agree, 



4x2 



