VOL. XXII.] PHILOSOPHICAL TRANSACTIOKS. 56Q 



the inward circle equal to the double tangent of the pole star from the pole, 

 the focal length of the object glass being made radius, as mentioned in the 

 description of the meridian instrument ; let the ring be divided into 24 hours 

 with their minutes numbered from the right hand towards the left, as in the 

 common nocturnals ; the eye glass must be equal in its diameter to the horary 

 ring ; but this perhaps will be thought loo chargeable, especially for such large 

 telescopes as above, which has made me think of this contrivance ; the eye 

 glass must lie in a broad index towards one end, which is to turn on a centre 

 pin in the centre of the glass, and consequently over the centre of the horary 

 ring, from which it must be equal to the distance of the focus of the eye glass ; 

 then let the tube be elevated to the height of the pole, and directed to the pole 

 star, till by turning the index through the eye glass, you perceive the star to 

 touch the horary ring on that side where the star in the great bear's rump lies, 

 or on the opposite to that in the hip of Cassiopea ; but the contrary, had 

 not the glass inverted the object; then bring one of the 12's to be in a per- 

 pendicular to the other by a plumb-line, so will the star stand at its horary 

 distance from the meridian ; or if the latitude of the place be unknown, by the 

 right ascension of the sun and star, the line of its coining to the meridian will 

 be easily obtained, and then the hour of the night found, will as easily give the 

 star's horary distance from the meridian ; then elevate the tube towards the star, 

 bringing the meridian, or'l2 and 12 into the plain of the perpendicular, turn the 

 glass about till you see the pole star stand at its horary diatance from the meri- 

 dian; CO will the instrument when fixed, show the horary distance throughout 

 the whole day, or as long as it remains in this position, by the apparent motion 

 of the star in the ring. The best time to fix the instrument will be when 

 this, or any of the other two stars abovementioned, are about 6 hours from 

 the meridian. Observe also, that the latitude of the place is now given with 

 the utmost preciseness ; for the axis of the glass lies now in the axis of the 

 world ; and if one of the sides of the tube be parallel thereto, as it ought to 

 be at the upper end, hang a line and plummet from the point of the suspension ; 

 find another point equal in distance to the length of the line, or a knot towards 

 the lower end, the distance from this knot to the former point will be only the 

 chord of the latitude, and if from the same edge of the index, another line 

 and plummet be hung towards the lower end of the tube, these two lines, when 

 at rest, will be in the plain of the meridian. 



This instrument may be made to show the hour with as much facility as a 

 clock or sun-dial, if the horary ring be made to move within a larger fixed one, 

 and the outward circle of the former be divided into the days of the month, 

 respect being had to the right ascension of the sun and star ; then by bringing 



VOL, IV. 4 D 



