WEATHER FORECASTS. 



315 



cells placed one upon the other and connected at the upper end of 

 the series to a system of magnifying levers that operate a pen. The 

 pen point rests upon a sheet of paper which is held on a cylinder 



Fig. 8. — The Triple Register. Front view. 



having clockwork within it to give it a slow rotation. The varying 

 pressure of the atmosphere causes the cells to contract and expand, 

 and this motion, transmitted through the levers, causes the pen to 

 trace a line on the graduated paper whose co-ordinates represent the 

 pressure of the atmosphere at any given time. 



The thermograph is somewhat like the barograph in principle. 

 The element that is affected by the temperature is a metal cell that 

 has the form of a curved and flattened tube, one end of which is 

 secured to the framework and the other end connected by a link to a 

 lever carrying a pen point. It has also a clock-driven cylinder with 

 its graduated sheet of paper. The tube is filled with alcohol, and, as 

 the liquid expands, it straightens the tube and moves the pen over the 

 paper, making an irregular line that represents the ever-changing 

 temperature. 



The maximum and minimum thermometers record the highest 

 and lowest temperatures respectively in the twenty-four hours. The 

 former is an ordinary mercurial thermometer with the addition of a 

 contraction in the tube just above the bulb. The heat forces the col- 

 umn of mercury past the contraction, but it can not return to the 

 bulb when the temperature lowers. The minimum thermometer is 

 an alcohol thermometer that has a colored glass float in the liquid. 

 When the alcohol contracts, the skin of the liquid carries the float 

 down with it; but, when the temperature rises, the float remains to 

 mark the lowest temperature. 



