OF DIALING. 357 



cording to the sun's declination for their respective days, I.ECT 

 and on the other side the last sis months. And against ^^^ 

 the clays on which the sun enters the signs, there are 

 straight lines drawn upon the semicircle, with the cha- 

 racters of the signs marked upon them. There is a 

 black line drawn along the middle of the upright edge 

 vf the quadrant, over which hangs a thread (H), with 

 its plummet (i) for levelling the instrument. 



X B. From the 23d of September to the 20th of 

 March, the upper surface of the circle must touch both 

 the center C of the semicircle, and the line of T and ^ ; 

 and from the 20th of March to the 23d of September, 

 the lower surface of the circle must touch that center 

 and line. 



To find the time of the day by this dial. Having set 

 it on a level place in sun-shine, and adjusted it by tire 

 levelling screws k and /, until the plumb-line hangs over 

 the black line upon the edge of the quadrant, and pa- 

 rallel to the said edge ; move the semicircle in the qua- 

 drant, until the line of <Y> and =0= (where the circle 

 touches) comes to the latitude of your place in the quad- 

 rant : then, turn the whole meridional plane B D, with 

 its circle G, upon the horizontal plane A, until the edge 

 of the shadow of the circle falls precisely on the day of 

 the month in the semicircle ;"* and then, the meridional 



Note 110. Since the plane of the dial, when the instrument is recti- 

 fied, is parallel to the equinoctial plane, and the declination of the sun 

 may be considered to remain the same whilst he continues above the 

 horizon, it follows, that the point where the shadow of the dial-plate 

 falls upon the edge E F on any particular day, will not alter its posi- 

 tion during that day. Now the angle formed by a line drawn from 

 that point to any point in the edge of the dial-plate with the dial plate 

 itself, is evidently equal to the declination of the sun on the given 

 lay. But the tangents of the sun's declinations to the radius of the 

 dial-plate, are marked upon this edge with the corresponding day of 

 the month. Consequently, the dial-plate being elevated to the proper 

 angle above the horizontal plane ; if the dial be placed north and 

 south, the shadow of the edge of the dial-plate will fall on the day of 



