ASTRONOMICAL INSTRUMENTS. 



137 



half to three inches in diame- Fi s- 139 - 



ter set in brass mounting is 

 supported ; the focus of the 

 lens corresponding to the dis- 

 tance of the touch-hole of the 

 cannon. On one of the quad- 

 rants there is marked a scale, 

 showing the altitude the lens 

 is to be set for every day in 

 the year, and corresponding 

 to the declination of the sun. 

 In setting the dial it should 

 be as near horizontal as possible, and the 12 o'clock line 

 north and south; when it will be found, that at 12 o'clock 

 the sun will shine through the lens and fall exactly on the 

 priming of the cannon, and the explosion take place. 



Price, 7J in. diam. $8.50; 9J in. diam. $12.50. 



Fig. 140. 



Universal Ring Dial. The Universal or Astronomical 

 Ring Dial (Fig. 140, as above,) is an instrument which 

 serves to find the hour of the day in any part of the earth ; 

 it consists of two rinses or flat circles, from 4 to G inches in 



