=Voluntary Weather Observers.= There are more than three thousand voluntary or coöperating observers in the U. S. Weather Bureau. They receive no compensation other than the publications of the Bureau. They are required to read their instruments but once each day, as maximum and minimum thermometers record the highest and the lowest temperatures since they were last read and set. About sunset is the most satisfactory time for making the readings, since the thermometers will then show both the extremes for the past twenty-four hours. As a rule but one voluntary observer is accepted for a county. They are furnished without charge with maximum and minimum thermometers, instrument shelters and rain gauges, but not with wind vanes, anemometers for recording direction and velocity of wind, or barometers. But those who desire to become expert in forecasting the weather, as all may who study the chapter on forecasting, should equip themselves with an aneroid barometer, so that they may note the changes in the pressure of the air. [Illustration: FIG. 5.—Comparison of the Thermometer Scales.] COMPARISON OF THERMOMETER SCALES A little study of the accompanying information and diagram will enable any one to form a clear idea of the various thermometer scales and to convert temperatures from one scale to another. _Table of fixed points._ -----------+----------------+----------------+-----------------+ Scale. | Temperature of | Temperature of | No. of degrees | | melting ice. | boiling water. | between melting | | | | ice and boiling | | | | water. | -----------+----------------+----------------+-----------------+ Centigrade | 0 | 100 | 100 | Reaumur | 0 | 80 | 80 | Fahrenheit | 32 | 212 | 180 | -----------+----------------+----------------+-----------------+ Only Fahrenheit and Centigrade scales are in general use, and the accompanying plate is designed to enable observers to convert temperature readings from one scale to the other without resorting to a mathematical formula. For accurate and precise reductions between the different scales the following rules should be used: 1. To convert Fahrenheit to Centigrade: Subtract 32 and multiply by five ninths. 2. To convert Centigrade to Fahrenheit: Multiply by nine fifths and add 32. 3. To convert Fahrenheit to Reaumur: Subtract 32 and multiply by four ninths. 4. To convert Reaumur to Fahrenheit: Multiply by nine fourths and add 32. 5. To convert Centigrade to Reaumur: Multiply by four fifths. 6. To convert Reaumur to Centigrade: Multiply by five fourths.