Page:Popular Science Monthly Volume 53.djvu/334

318 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.



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