358 FERGUSON'S LECTURES. 



LECT. plane will be due north and south, the axis E F will be 



-y 



^^^ parallel to the axis of the world, and will cast a shadow 

 upon the true time of the day, among the hours on the 

 circle. 



IV. B. As, when the instrument is thus rectified, (he 

 quadrant and semicircle are in the plane of the meridian, 

 so the circle is then in the plane of the equinoctial. 

 Therefore, as the sun is above the equinoctial in sum- 

 mer (in northern latitudes) and below it in winter ; the 

 axis of the semicircle will cast a shadow on the hour of 

 the day, on the upper surface of the circle, from the 

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

 23d of September, to the 20th of March, the hour of the 

 day will be determined by the shadow of the semicircle, 

 upon the lower surface of the circle. In the former 

 case, the shadow of the circle falls upon the day of the 

 month, on the lower part of the diameter of the semi- 

 circle ; and in' the latter case, on the upper part. 



The method of laying down the months and signs 

 upon the semicircle, is as follows. Draw the right line 

 A C B, (fig. 5.) equal to the diameter of the semicircle 

 A D B, and cross it in the middle at right angles with 

 the line E C D, equal in length to A D B ; then E C 

 will be the radius of the circle FCG, which is the same 

 as that of the semicircle. Upon E, as a center, describe 

 the circle FCG, on which set off the arcs Ch and C i, 

 each equal to 23| degrees, and divide them accordingly 

 into that number, for the sun's declination. Then, lay- 

 ing the edge of a ruler over the center E, and also over 

 the sun's declination for every 111 fifth day of each 

 month (as in the card dial) mark the points on the dia- 

 meter A B of the semicircle from a to g, which are cut 



the month : and vice versa if the dial-plate be turned, until the shadow 

 of its edge falls upon the day of the month, then the dial will be set 

 truly north and south. 



Note 111. The intermediate days may be drawn in by hand, if 'he 

 space be large enough to contain them. Note ly the Author, 



