The thermometer, as the reader knows, measures the temperature of the air; and in all readings of the barometer the changes in temperature have to be taken into account. The weather observers use two kinds of thermometers, the dry and the wet bulb. The dry bulb is the ordinary form, which every one knows, and is used to measure heat and cold. The wet has the bulb wrapped in some absorbent material, which is kept soaked with water. Now you know, without my telling you, that the water will cool the bulb, and hence the wet-bulb thermometer will stand lower than the dry. That cold is caused by evaporation, and the evaporating power of the atmosphere depends upon the amount of moisture there is in the air. So you at once see that the difference between the readings of the wet and dry bulb thermometers indicates the amount of moisture in the air. This amount the observers express in percentages of 100; and thus we read of "humidity, 60 per cent." Under ordinary circumstances, when the humidity gets close to 100, the point at which the air is soaked with moisture, it is going to rain. The temperature, however, and also the wind, have a good deal to do with this. The form in which the weather observers use these two thermometers is called the whirling psychrometer. The two instruments are put on the end of an arm, which is fixed on an axle turned by a crank. The observer whirls this around a few times before reading the instrument, for the purpose of making the air act freely on the two bulbs. The direction of the wind, as every one knows, is shown by a weather vane. Those which are used by the observing stations, however, have an attachment which automatically records on a sheet of paper every variation of the vane, so that the office has an account of the smallest changes of the wind during the twenty-four hours. The speed of the wind is measured by the anemometer. This consists of four half-spheres at the end of four horizontal arms, which centre on an upright axle. The force of the wind causes the arms to revolve, and it has been found that 500 revolutions equal one mile. If the arms revolve 3000 times in an hour, the wind is blowing six miles an hour. The revolving of the upright axle operates a contrivance by which the speed of the wind for every minute in the day is recorded. The amount of rain which falls is measured in a way which shows what the depth of water would be on a level surface if it did not, in the natural order of things, run off. The rain is caught in a funnel 8-1/2 inches in diameter, so placed as to be protected from all gusts of wind. The record is made in five-hundredths of an inch.