or DIALING. 339 



with the meridian plaue of the place (which marks the 

 hour-line of XII) are called hour-circles ; and their in- 

 tersections with the plane of the dial, are called hour- 

 lines. 



In all declining dials, the substile makes an angle 

 with the hour-line of XII; and this angle is called the 

 distance of the substile from the meridian. 



The declining plane's difference of longitude, is the 

 angle formed at the intersection of the stile and plane of 

 the dial, by two meridians ; one of which passes 

 through the hour-line of XII, and the other through the 

 substile. 



This much being premised concerning dials in general, 

 we shall now proceed to explain the different methods of their 

 construction. 



If the whole earth a Pep were transparent and hol-Theuni- 

 low, (plate J|, fig. 2.) like a sphere of glass, and had i 



equator divided into 24 equal parts by so many meri-n which 

 dian semicircles, a, b, c, d, e, f, g, &c. one of which is the depend*. 

 geographical meridian of any given place, as London, 

 (which is supposed to be at the point a ;) and if the 

 hours of XII were marked at the equator, both upon 

 thaieridian and the opposite one, and all the rest of 

 the hours in order on the rest of the meridians, those 

 meridians would be the hour-circles of London : then, 

 if the sphere had an opaque axis, as P E p, terminating 

 in the poles P and p, the shadow of the axis would 

 fall upon every particular meridian and hour, when the 

 sun came to the plane of the opposite meridian, and 

 would consequently shew the time at London, and at all 

 other places on the meridian of London. 



If this sphere was cut through the middle by a solid Horizontal 

 plane A B C D, in the rational horizon of London, one 

 half of the axis E P would be above the plane, and the 

 other half below it ; and if straight lines were drawn 

 from the center of the plane, to those points v. here its 



Z 2 



