The common thermometer was long ago discredited as a means of measuring atmospheric comfort. Then, for a time, the wet-bulb thermometer had its day, and its indications were once published on a large scale in this country as representing the “sensible temperature,” or temperature that we actually feel. The wet-bulb thermometer is cooled by evaporation below the air temperature, except when the air is saturated with moisture, and may therefore give a rough indication of the temperature acquired by the skin when moistened with perspiration. The temperature of the skin is not, however, an accurate indication of our feelings of heat or cold, nor is it a satisfactory guide to the physiological effects of atmospheric conditions. Several instruments have been devised for the purpose of measuring the cooling power of the air, to which the bodily mechanism must respond in order to maintain a uniform temperature within. One of these was invented nearly half a century ago by J. W. Osborne. A porous cylinder was filled with warm water, and an agitator, driven by clockwork, kept the water in the cylinder at uniform temperature at any given moment. The rate at which heat was lost from the wet surface of the cylinder was determined by a thermometer, having its bulb immersed in the water, and a stop watch. A different plan was adopted by A. Piche, in an instrument which he called the _deperditometer_, and which was supposed to imitate more closely the behavior of the body. A porous vessel is filled with water, the temperature of which is kept constantly at “blood heat” (98.6 degrees F.) by a gas jet provided with an automatic regulator. The amount of gas burned in a given time is then supposed to measure the cooling power of the atmosphere as it affects humanity. J. R. Milne’s _psuchrainometer_ is constructed on the same principle, but heat is supplied and measured electrically. Among many other instruments of this class, one that now enjoys special favor is the _katathermometer_, devised by Prof. Leonard Hill, in England. This consists of a pair of large-bulbed spirit thermometers, one of which has its bulb covered with fine cotton mesh. To use the instrument, the bulbs are immersed in water at about 150 degrees F. until the spirit rises to the top of the thermometer tubes. The excess of water is then jerked off the wet bulb, and the other bulb is dried. The instruments are finally suspended in the air, and the rate of cooling from 110 to 100 degrees, or from 100 to 90 degrees, is taken with a stop watch. The dry-bulb measurements are supposed to show how fast the human body loses heat at its surface by radiation and convection, while the wet-bulb measurements also take account of evaporation.