D 1 A 



Equatorial Dial. 



In this Dial, since the sun moves uniformly 15<> per hour, the ,/s. at P, 

 and consequently the arcs of the circle Q E, which measure them, will 

 increase uniformly. Hence we have only to take from Q the area 15, 

 30, 45, &c., and they will be 1 o'clock, 2 o'clock, 3 o'clock, &c., marks. 

 This Dial, unless graduated on both sides, will only shew the hours for 

 the six summer months, viz. from the vernal to the autumnal equinoxes. 



Horizontal Dial. 



Here the arcs R 1', 1' 2' &c. are not equal, but must be calculated by 

 the resolution of the right /d. As. P R 1', PR 2', &c., where RP 1' = 

 15, R P 2' 300, & c>> then we shall have 



tan. R 1' = sin. lat. X tan. 150. 

 tan. R 2' = sin. lat X tan. 2 X 15. 



&c. &c. 



This Dial shews the hour throughout the year, whenever the sun is 

 above the horizon. In order to fix a horizontal dial, find the time by the 

 sun's alt. when it is at or near the solstices, and set a well regulated 

 watch to that time ; then when the watch shews 12 o'clock, at that iu- 

 stant set the dial to 12 o'clock, and it stands right. 



Vertical North and South Dial's. 



Here to find the arcs N 1", N2", &c., we have in the right ^d, A 

 P' N 1", 



tan. N 1" = cosin. lat. X tan. 15o. 

 tan. N 2" zr cos. lat. X tan. 2 X 15<>. 

 &c. &e. 



If P be the North Pole, this represents a South Dial. The construc- 

 tion for the Vertical North Dial is nearly the same. In this Dial the 

 number of hours shewn in a day can never exceed twelve, which is the 

 case at both the equinoxes ; at any other season of the year, the num. 

 ber of hours shewn is less. 



To find whether a wall be full south for a vertical south Dial, erect a 

 gnomon perpendicular to it, and hang a plumb line from it ; then when 

 the watch shews 12, if the shadow of the gnomon coincide with tha 

 plumb line, the wall is full south. 



