-1859] WRITINGS OF JOSEPH HENRY. 241 



quantity of vapor which the air could contain at the given 

 temperature, to that which it actually contains. According to 

 this definition two places would be equally damp which are 

 both half saturated with vapor, though the abstract quantity of 

 vapor in the one case may be many times that of the other. 

 Thus in winter when the temperature is very low and the 

 absolute quantity of vapor in the air is exceedingly small, 

 the air may have a maximum of dampness, that is to say, a 

 very great relative humidity. Although this method of 

 establishing the relative humidity of different places may 

 correspond with variations in different phenomena, yet there 

 are some effects which appear to depend not on the relative 

 but on the absolute amount of humidity in the air. The 

 conducting capacity for electricity (for example) appears to in- 

 crease with the absolute amount of vapor in the air, and hence 

 experiments with the electrical machine succeed much better 

 in winter than in summer, though the relative humidity in 

 both cases may be the same. Again, since the temperature of 

 our bodies is about 98°, and as this may be regarded as the 

 temperature of an evaporating surface, the difference of ten- 

 sion of vapor from the pores of the skin and that in the air 

 must be very different in winter and in summer; and hence 

 in the latter case, when the dew-point approaches the temper- 

 ature of the body, we experience the sensation of the close- 

 ness and sultriness of the atmosphere. 



On the other hand the intense cold which is felt on the 

 Western plains in winter is due principally to the rapid evapo- 

 ration from the pores of the skin — a result which can only 

 be guarded against by a covering of close texture, such as 

 the prepared skins of animals. In this connection we may 

 mention a fact, which at first sight might appear to militate 

 against the usages of civilized and refined life, namely, that 

 dirt and grease are great protectors of the skin against in- 

 clement weather, and therefore, says Mr. Galton, "the leader 

 of a party should not be too exacting as to the appearance 

 of his less warmly clad followers." Daily washing, if not 

 followed by oiling, must be compensated by warmer clothing. 

 A savage never washes himself in cold weather unless he 



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