340 



FERGUSON'S LECtfURES. 



I, EOT 

 X. 



Vertical 



dial. 



Inclining 

 and re- 

 clining 

 dials. 



Declining 

 dials. 



circumference is cut by the hour-circles of the sphere, 

 , those lines would be the hour-lines of a horizontal dial 

 for London : for the shadow of the axis would fall upon 

 each particular hour-line of the dial, when it fell upon 

 die like hour circle of the sphere. 



If the plane which cuts the sphere be upright, as 

 AFCG, (plate 4, fig. 3.) touching the given place 

 (London,) at JF, and directly facing the meridian of Lon- 

 don, it will then become the plane of an erect direct 

 south dial : and if right lines be drawn from its center 

 E, to those points of its circumference where the hour- 

 circles of the sphere cut it, these will be the hour-lines 

 ot a vertical or direct south dial for London, to which the 

 hours are to be set as in the figure (contrary to those on ;i 

 horizontal dial) and the lower half Ep of the axis will 

 cast a shadow on the hour of the day in this dial, ar the 

 same time that it would fall upon the like hour-circle of 

 the sphere, if the dial-plane was not in the way. 



If the plane (still facing the meridian) be made to in- 

 cline, or recline, by any given number of degrees, the 

 hour-circles of the sphere will still cut the edge of the 

 plane in those points to which the hour-lines must be 

 drawn straight from the center ; and the axis of the 

 sphere will cast a shadow on these lines at the respec- 

 tive hours. The like will still hold, if the plane be 

 made to decline by any given number of degrees from 

 the meridian, towards the east or west : provided the 

 declination be less than 90 degrees, or the reclination 

 be less than the co-latitude of the place : and the axis of 

 the sphere will be a gnomon, or stile, for the dial. But 

 it cannot be a gnomon, when the declination is quite 90 

 degrees, nor when the inclination is equal to the co-lati- 

 tude ; because in these two cases, the axis has no eleva- 

 tion above the plane of the dial. 



And thus it appears, that the plane of every dial re- 

 presents the plane of some great circle upon the earth ; 



