=94. Air thermometers.= The accurate measurement of temperature requires standard thermometers. Reasonably accurate instruments may be standardized by determining their error, but they are extremely unsatisfactory in practice, since they result in a serious waste of time. Accurate thermometers which read to the degree are entirely serviceable as a rule, but instruments which read to a fraction of a degree are often very much to be desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are delicate and require careful handling, but even in class work this has proved to be an advantage rather than otherwise. In making readings of air temperatures with such thermometers, constant precautions must be taken to expose the bulb directly to the wind and to keep it away from the hand and person. =95. Soil thermometers.= The thermometer described above has been used extensively for soil temperatures. The determination of the latter is conveniently combined with the taking of soil samples, by using the hole for a temperature reading. When carefully covered, these holes can be used from day to day throughout the season without appreciable error, even in gravel soils. Repeated tests of this have been made by simultaneous readings in permanent and newly made holes, and the results have always been the same. It has even been found that the error is usually less than 1 degree when the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in the fact that the thermometer must be raised to make the reading. With a little practice, however, the top of the column of mercury may be raised to the surface and read before the change of temperature can react upon it. This is especially important in very moist or wet soils where the bulb becomes coated with a film of moisture. This evaporates when the bulb is brought into the air, and after a moment or two the mercury slowly falls. [Illustration: Fig. 16. Soil thermometer]