The difficulty of obtaining the true temperature of the air from a balloon is great, and without special precautions the observations give the conditions of the free air even less well than do observations on mountain summits. During a rapid ascent the air is carried up in the balloon basket like water in a well-bucket, and since the balloon drifts with the wind it is relatively in a calm, so that there is no circulation of air; the thermometers, even when screened from direct sunshine, are affected by radiation from the heated gas-bag above, and moreover they are not sufficiently sensitive to follow the changing temperature of the air strata so quickly traversed by the balloon. The aneroid barometer, from which the height of the balloon is calculated, cannot respond to rapid changes of pressure; consequently there is a double source of error in determining the height at which the temperature is measured. Ordinarily, the temperature of the air may be obtained quite accurately by slinging in a circle a thermometer attached to a cord, even though this is done in sunshine. During two balloon ascents by the writer, a sling thermometer was found in extreme cases to read 14∞ lower than was recorded by automatic instruments, hung in their usual position from the ring of the balloon. The sling thermometer, however, is influenced by intense insolation, and moreover cannot be swung far enough outside the basket of a balloon to insure good results. The standard instrument for obtaining the temperature of the air under all conditions, adopted for international use in 1898, is a modification of that used by Welsh forty-five years before. This instrument, which is the invention of Dr. Assmann of Berlin, is called the aspiration thermometer, and is designed to prevent the casing surrounding the thermometer from being heated by insolation or conduction, and to insure a flow of air past the thermometer bulbs. [Illustration: FIG. 3.--German Balloon equipped for Meteorological Observations.]