370 FERGUSON'S LECTURES. 



LECTURE XT. 



OP DIALING. 



HAVING shewn in the preceding Lecture how to make 

 sun-dials by the assistance of a good % globe, or of a 

 dialing scale, we shall now proceed to the method of 

 constructing dials arithmetically ; which will be more 

 agreeable to those who have learned the elements of 

 trigonometry, because globes and scales can never be so 

 accurate as the logarithms, in finding the angular dis 

 tances of the hours. Yet, as a globe may be found 

 exact enough for some other requisites in dialing, we 

 shall take it in occasionally. 



The construction of sun-dials on all planes whatever, 

 may be included in one general rule : intelligible, if that 

 of a horizontal dial for any given latitude be well un- 

 derstood. For there is no plane, however obliquely 

 situated with respect to any given place, but what is 

 parallel to the horizon of some other place ; and there- 

 fore, if we can find that other place by a problem on the 

 terrestrial globe, or by a trigonometrical calculation, 

 and construct a horizontal dial for it ; that dial, applied 

 to the plane where it is to serve, will be a true dial for 

 that place. Thus, an erect direct south dial in 51J de- 

 grees north latitude, would be a horizontal dial on the 

 same meridian, 90 degrees southward of 51i degrees 

 north latitude ; which falls in with 38| degrees of south 

 latitude. But if the upright plane declines from facing 

 the south at the given place, it would still be a horizon- 

 tal plane 90 degrees from that place ; but for a different 

 longitude : which would alter the reckoning of the hours 

 accordingly. 



