136 THE POPULAR SCIENCE MONTHLY 



mer often exceeding 90° F., and minimum temijeratnres in winter fre- 

 quently going below 0° F. San Francisco, on the other hand, has a 

 semi-tropical climate, with temperatures of 90° F. or over occurring 

 but two or three times in a year, and minimum temperatures below 

 40° F. being equally rare. In addition, the climates of the two cities 

 differ greatly in respect to the amount and duration of sunshine, cloudi- 

 ness, rainfall, relative humidity, wind velocity and direction, and the 

 various other elements which constitute climate. The mean annual 

 temperature is therefore an inadequate indication of climatic condi- 

 tions, and can not alone serve as a basis of comparison. 



Too much emphasis is also placed upon the temperature itself — 

 our feeling of comfort is by no means entirely dependent upon the read- 

 ing of the dry-bulb thermometer. An ideal curve of comfort might 

 show but little resemblence to the thermograph trace. Relative humid- 

 ity is so important a contributory factor that the wet-bulb rather than 

 the dry-bulb thermometer is often the better indicator. The feeling 

 produced by a temperature of 100° F. experienced in southern Arizona 

 is wholly unlike that accompanying a similar temperature in an eastern 

 city, the difference being due primarily to the marked difference in 

 relative humidity. Other factors also affect one's feeling of comfort, 

 such as sunshine, wind velocity, barometric pressure, and atmospheric 

 electricity. Some time it may be possible to give correct relative weights 

 to each of these factors in determining their effect upon the man in 

 perfect health. While temperature doubtless will receive the greatest 

 weight, the other factors are by no means negligible. 



Night air is occasionally referred to as though it is different from 

 day air, and convalescents are sometimes urged to avoid it as danger- 

 ous. While there are obvious physical differences between night air 

 and day air there is little diurnal change in chemical composition. 

 Atmospheric air is a physical mixture which when perfectly dry con- 

 sists principally of nitrogen, oxygen, argon, and carbon dioxide, in 

 which the relative proportions remain fairly constant, and in which the 

 first two named constitute more than 99 per cent, by volume. Up to 

 heights greater than the summits of the highest mountains the per- 

 centage of oxygen, an element necessary in the respiration of both plants 

 and animals, shows no appreciable variation. Carbon dioxide, however, 

 which forms but .03 per cent, by volume, or .05 per cent, by weight, of 

 the air, shows both an annual and a diurnal variation. By volume it is 

 23 per cent, greater in summer than in winter, and is 12 per cent, 

 greater at night than during the day. Since carbon dioxide does not 

 become dangerous until it constitutes considerably more than 1 per cent, 

 of the air we breathe, the change from day to night can not account for 

 the supposedly offensive feature of night air. Water vapor, which never 

 exceeds 4 per cent, by volume of atmospheric air, is important as far as 



