696 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 749. 



Above this meridian semicircle is fastened a circular plate, on which are affixed 

 2 Other circular plates ff, moveable on each other, and called the equatorial 

 plates; one of them, representing the plane of the equator, is divided into twice 

 \2 hours, and these are subdivided into every 10 minutes of time. This plate 

 is moved by a handle g, called the equatorial handle, and has a nonius index for 

 showing every minute. 



Above this equatorial plate is a semicircle hh, called the declination semicircle, 

 as representing the half of a circle of declination, or horary circle, and is divided 

 into twice gO°, being moved by the handle k, called the declination handle. It 

 has also a nonius index for subdividing into every 3 minutes. 



Above this declination semicircle is fastened a reflecting telescope ll, of the 

 Gregorian construction, the focal length of its great speculum being 18 inches. 



To adjust the instrument for observation, the first thing is to make the hori- 

 zontal plates level or horizontal, by means of the 2 spirit levels, and the 4 screws 

 in the cross pedestal. This done, move the meridian semicircle, by means of 

 the meridian handle, so as to raise the equatorial plates to the elevation of the 

 equator of the place; which is equal to the complement of the latitude, and 

 which, if not known, may likewise be found by this instrument, as shall be 

 afterwards shown. And thus the instrument is ready for observation. 



To Jind the Hour of the Day, and Meridian of the Place. — First find, from 

 astronomical tables, the sun's declination for the day, and for that particular 

 time of the day; then set the declination semicircle to the declination of the 

 sun, taking particular notice whether it is north or south, and set the declination 

 semicircle accordingly. Then turn about the horizontal handle^ and the equa- 

 torial handle, both at the same time, till the sun be precisely concentrical with 

 the field of the telescope. If there is a clock or watch at hand, mark that in- 

 stant of time; and by looking on the equatorial plate, and nonius index, you 

 will find the hour and minute of the day, which comparing with the time shown 

 by the clock or watch, shows how much either of them differs from the sun. In 

 this manner you find the hour of the day. 



Now, in order to find the meridian of the place, and consequently to have a 

 mark, by which you may always know the meridian again, first move the equa- 

 torial plate, by means of the equatorial handle, till the meridian of the plate, or 

 hour line of 12, be in the middle of the nonius index; and then, by turning 

 about the declination handle till the telescope comes down to the horizon, you 

 observe the place or point which is then in the middle of the field of the teles- 

 cope; and a supposed line drawn from the centre of this field to that point in the 

 horizon, is the meridian line. The best time of the day for making this obser- 

 vation for finding the meridian, is about 3 hours before noon, or as much after 

 noon. The meridian of the place may be found by this method so exact, that 



