The essential points of any anemometer are that it should be correctly calibrated and should read accurately over the whole range through which it is to be used, and that the calibration should not alter with time. The actual calibration of an anemometer by a direct comparison of its readings with its rate of movement through the air is a matter of very great difficulty, and the various methods by which it has been done will be considered later, but when it has once been cairied out for one anemometer all others can be easily calibrated from that one by comparison.