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.
In order to connect the readings of this device with human sensations, Hill and his collaborators have used the instrument under a great variety of atmospheric conditions, both indoors and out, and compared the readings with independent estimates of comfort and discomfort. On an ideal summer day the “wet kata” fell from 110 to 100 in 25 seconds, and the “dry kata” in 85 seconds. Indoors at the seaside in summer, under comfortable conditions, the readings were 50 seconds and 140 seconds, respectively. A large number of other readings, taken under different conditions in various parts of the word, have been published. The katathermometer is now used in both Great Britain and the United States in the study of ventilation problems, and has acquired a rather extensive literature. According to its inventor, “the heating and ventilation of rooms should be arranged so that the wet-bulb falls from 100 to 90 degrees in about one minute, and the dry-bulb in about three minutes.”
A Weather Bureau Kiosk, in Union Square, San Francisco.
(Photograph, U. S. Weather Bureau.)
The United States Weather Bureau Station, Observatory Type, at Peoria, Illinois. (Photograph, U. S. Weather Bureau.)
Regardless of the merits of these particular instruments, it is certain that the cooling power of the air—which is quite a different thing from the temperature of the air—is a very important factor in determining our comfort and our health. Within certain limits the body can easily adjust itself to changes in the cooling power of the air; within wider limits the adjustment is effected with difficulty, and we experience discomfort and possibly suffer in health; and finally there are extreme conditions, in either direction, to which adjustment is not possible; the internal temperature is then either lowered or raised, as the case may be, and a comparatively small change of this sort is fatal; i. e., death results by chilling or by heat stroke.
Photo U. S. Weather Bureau