For practical purposes there are no better hygrometers than those that depend on the hygroscopic properties of certain organic substances, such as hair and catgut, and it has been shown by Sresnewsky that the alteration in length of a hair caused by its absorption of moisture is directly proportional to the natural logarithm of the degree of relative humidity, so that such instruments can be graduated for use as scientific instruments. Rapidity of evaporation is, however, proportional not to the relative humidity, but to the difference of the tension of watery vapour present in the air with that of its tension when saturated--in other words, the difference of tension of watery vapour at the temperatures of the dry and wet bulb thermometers, a form of expression which admits of degrees of humidity at different temperatures to be directly compared. In practice, however, this datum is rarely to be found in climatic tables, which is of the less importance, as in its effects on our organisation a difference of 2 or 3 mm. of mercury, from the pressure of saturation at a low temperature, will give a pleasant sensation of dryness, while at a high temperature the same deficit of pressure would be felt intolerably close and sultry. Extreme conditions of either humidity or dryness are, of course, alike unhealthy, though much of the respiratory irritation ascribed to too dry air is, I believe, more truly referable to the dust which usually accompanies such atmospheric conditions; but in any case, there can be no doubt that alike in hot and cold climates it is far healthier for the air to be too dry than too moist. With the effects of damp cold we are all of us only too well acquainted in England, and those who have experienced the effects of damp heat will never need being reminded of its debilitating effects. Fortunately, however, relative humidities exceeding 80 per cent. are but rarely to be found accompanied with really high air temperatures, and are seldom met with except in localities blessed with a copious rainfall, which by cooling the air goes far to render matters tolerable.