Page:Popular Science Monthly Volume 53.djvu/331

Rh 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



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 column 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.