628 PHILOSOPHICAL TRANSACTIONS. [aNNO 1703, 



through slits in the exterior tube, and are confined between nuts, in the same 

 manner as the object-lens. This construction has many advantages over that be- 

 fore described. 



To obviate the necessity of the gold tongue, besides the moveable wire in the 

 field of the microscope, there is a 'id, which may be considered as fixed, havino- 

 only a small motion for its adjustment. When the instrument is adjusted, and 

 the index belonging to the micrometer-screw stands at the zero on its circle, the 

 moveable wire cutting one of the dots on the limb of the instrument, this fixed 

 wire must be made to bisect the next dot; as by this means it may be perceived 

 at any time, whether the relative position of the wire has varied. By graduating 

 the limb of the instrument to every JO', instead of 15, we are enabled to mea- 

 sure by the micrometer-screw, not only the excess of the measured angle above 

 any of the JO', but also its complement to the next division on the circle, and 

 so to correct any small inequality which may happen among the divisions. 



Though it might have been reasonably supposed, that the angles of the tri- 

 angle King's Arbour, Hampton Poor-house, and St. Ann's Hill, had been ob- 

 served with sufficient accuracy in 1787, yet that this operation might not rest on 

 data afforded by any former one, it was considered as proper to determine them 

 by our own instrument. The first station to which the instrument was taken 

 this year was Hanger Hill, because it was found on examination, that the part 

 of the stage which had been left at Shepperton was much damaged, and stood 

 in need of considei'able repair. It was however soon fitted for use, and a new 

 tent for the top having been provided, the half stage was erected over the pipe 

 at St. Ann's Hill, to which from Hanger Hill the instrument was conveyed. 

 Here, as well as at the other stations where the stage was used, a plumb-line 

 was let fall from the axis of the instrument over the point marking the station, 

 being sheltered from the wind by a wooden trough. In the use of the lialf stage, 

 the instrument was sufficiently steady when the wind blew moderately; but from 

 the crazy state of the lower part, it was only by watching for moments particu- 

 larly calm, that satisfactory observations could be made when the whole of it 

 was used. 



The consequent observations will sufficiently explain the detail of this 

 year's operations, which are given in the order of time in which they were 

 made. It may be noticed, that most of the angles have been observed more 

 than once; indeed it was a position which we laid down on our commencing 

 this business, atid which, as far as circumstances would admit, has since been 

 adhered to, namely, that of observing the angles on different arcs. When 

 staffs were erected, which was generally the case when the stations were not 

 more remote than 15 miles, the angles were repeated till their truth became 

 certain, and the same was also done when angles were determined by the lamps; 



