VOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 30Q 



each other. The perpendicular wires are 5, at 35 seconds of time distance in the 

 equator ; and are adjustable horizontally for collimation by a screw. The hori- 

 zontal wires are 3. at about 15' of a degree asunder ; placed so as just not to 

 touch, but to pass clear of the other wires ; and they are adjustable in collimation 

 by another screw peculiar to them. My reason for having 3 horizontal wires, and 

 at about that distance, was, that after having ascertained what the difference is, I 

 might observe the lower limb of the sun or moon at the one, and the upper limb 

 at the other of the extreme wires, without much altering the elevation of the 

 telescope, and removing the centre of the object, or preceding and subsequent 

 limbs of the sun or moon, far out of the centre of the field The divisions on 

 the circle itself come now to be spoken of. They were done by hand ; and have 

 been executed with great care. The original divisions are by dots or points, at 

 every 10 minutes. Within is another row, by strokes or cuts ; laid oft' from the 

 points to every 10' also. 



That the numbering of the degrees might coincide with this idea, I considered, 

 that the figures should be made to appear erect in the microscopes, in every posi- 

 tion of the telescope, and that they should be reckoned backwards. To effect 

 this, they ought to be reckoned backwards in themselves, but to stand the con- 

 trary way, or inverted in reality. This would be different in the two microscopes, 

 in respect of the centre of the circle ; but that could create no difficulty. For 

 since the 2 quadrants nearest to the object-end of the telescope, would always be 

 those coming under the examination of microscope a ; and the '2 nearest to the 

 eye-end, those to be observed at microscope b ; they might be figured accordingly. 

 Hence, supposing the instrument placed in the meridian, with the graduated 

 face turned towards the east; if, when the telescope is horizontal and points to 

 the south, the upper quadrant nearest to the object-end, be numbered from that 

 end from 1 to 90°, with the heads of the figures towards the centre of the in- 

 strument ; and the other upper quadrant be numbered from the eye-end, with 

 the feet of the figures towards the centre ; they both would give the zenith dis- 

 tances of the objects observed. The former, at microscope a, while the tele- 

 scope points to the south of the zenith ; the latter at microscope b, when you are 

 observing towards the north. 



In the progress of this transit circle, when the divisions came to be examined 

 in their proper position, as to the truth of the opposite dots being exactly in the 

 diameter of the circle, an error was discovered, which occasioned a great deal of 

 trouble, and much loss of time. When the microscopes had been adjusted with 

 care, after turning the circle one way, they continued true, and the same dots 

 showed themselves to be perfectly in the diameter, however often the circle were 

 turned the same way round : but on one or more revolutions the contrary way, 

 the same dots ceased to appear true. This, it was thought, could arise only 



