136 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



having a style, or gnomon, which is usually the edge of a 

 plate of metal, or a cylindrical rod, fixed at an angle equal 

 to the latitude of the place where the dial is to be used. 



The face of the dial is divided into hours, from 5 to 12 

 on one side of the gnomon, and from 12 to 7 on the other; 

 there are intermediate divisions for half, quarter hours, and 

 minutes ; also, the principal points of the compass are drawn 

 on it. Fig. 137 represents the face of the dial, and Fig. 

 138 the gnomon and side view. 



Fig. 137. 



Fig 138. 



The time indicated by the sun dial, is solar or true time, 

 and agrees with mean time, or that shoAvn by a clock, only 

 on four different days in the year. To find the mean time, 

 it is necessary to apply a correction, called the equation of 

 time. 



Price, in brass, for any lat. required, 6 in. diam. $4.50. 



" ' " " 7 " " $5.50. 



tt ( < 3 < $7.50. 



" in marble, " " " 7^ " " $4.00. 



11 " " $6.50. 



13 " " $8.00. 



" " " " " 18 " " $18.00. 



The Sun Dial with Lens and Cannon. (Fig. 139, next 

 page.) This instrument is so arranged, that, the heat of the 

 sun falling through a lens, and being concentrated on the 

 touch-hole of a loaded cannon, it will precisely at noon be 

 fired. It consists of a slab of marble, having a sun dial of 

 the usual construction on one portion, and on another part a 

 brass cannon, at the sides of which there are two quadrants 

 with movable arms, by which a lens of about two and a 



