318 



POPULAR SCIENCE MONTHLY. 



tion of a pen acting on the paper. The number of jogs in an hour 

 thus made in the line traced by the pen gives the velocity of the wind. 

 The ninth and tenth pins are connected, so that one long jog occurs 

 in the record for every ten miles, making it easy to count the total. 



Fig. 11. — The Sunshine Becordeb. 



The sunshine recorder is constructed on the principle of the 

 differential thermometer. Inside of a vacuum tube is a tube having a 

 bulb formed on each end, and the inner tube extends into the lower 

 bulb nearly to its bottom. Both bulbs contain air; and the lower 

 one, which is coated with lampblack, has a quantity of mercury in its 

 lower part. The mercury also extends up into the tube. Two wires 

 enter the opposite sides of the inner tube between the bulbs, and these 

 wires form part of the electrical circuit of the one of the magnets 

 of the triple register which magnet occupies a side of the triple 

 register by itself. The armature of the magnet, through a pawl-and- 

 ratchet mechanism, gives the pen lever of this magnet a step-by- 

 step motion, first to one side and then to the other. This action takes 

 place when the sun, shining on the heat-absorbing lampblack, causes 

 the air and mercury in the lower bulb to expand and force the 

 mercury up the inner tube until it completes the electrical cir- 

 cuit between the wires in the inner tube. The clock breaks the 



