162 



G. S. Isham — Registering Solar 



equilibrium. This will be true whatever changes there may 

 be in the temperature of the air, or the atmospheric pressure. 

 If the sun, or any source of heat, act upon the black tube 

 the tension is increased, some mercury is forced out of the 

 tube, the weight becomes less, contact is made at III, the mag- 



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net Kl engages the wheel LI, the motor is started and the 

 carriage is moved out upon the beam until the balance is 

 restored, when contact is broken and the motor stopped. 



When the source of heat is removed the tube cools, tension 

 is decreased, amount of mercury is increased, equilibrium is 

 disturbed, contact at R is made, and motion of carriage re- 

 versed and continued until equilibrium is again restored. 



Secured to the nut G is a pen which moves with the nut 

 parallel to the axis of a cylinder O, rotated once in twenty-four 

 hours by the clock P. 



The following dimensions for parts give good results : tubes 

 f-inch inside diameter, 32*5 inches long; scale beam 30 inches. 



