The dew-point has been a puzzle to most persons.  Very few treatises explain it satisfactorily.  The definition just given, though explicit, is not quite enough.  For it will be perceived that an ordinary subtraction of the degrees of temperature on a wet thermometer, which had cooled down by evaporation, from the actual temperature indicated by a dry thermometer, will not give us the dew-point.

For example,—­if a free or dry thermometer indicates 63 deg., and the one with the wet bulb has by evaporation cooled down to 54 deg., the difference would be 9 deg..  The dew-point would not be 54 deg., but that degree to which the mercury would fall in the free thermometer, for the atmosphere to become saturated with the quantity of moisture then actually existing in it.  It would be 46.8 deg..

This dew-point, which figures so largely in all well-kept meteorological reports, is the key to many important conditions of the atmosphere, affecting health, vegetation, and climate.

It is found that the air at different degrees of heat has different degrees of elasticity, different degrees of tension, and different degrees of capacity to hold vapor.  Dalton, by a series of experiments with barometer-tubes, into which he introduced air and vapor at certain temperatures, found what its force was upon the mercurial column from degree to degree.  He also experimentally determined the ratio of the weight of moisture and of air, the former being five-eights of the latter,—­in other words, how many grains of moisture additional could be held by the air, advancing from degree to degree of temperature.  This being ascertained, a table of factors was constructed, in other words, a set of figures contrived, which should, by a multiplication of the subtracted difference between the range of the dry bulb and the wet bulb of the thermometers, furnish the amount of deduction from the former which would indicate the dew-point, or the point to which the mercury in the dry thermometer must fall to show how much more moisture the air could hold without its condensation.  These tables of factors have been constructed at the Greenwich Observatory, and are generally used.

The Hygrometer, invented by Mr. Daniell, gives the dew-point by inspection.

It is an error to suppose that dew falls like rain from the air; it forms on the body which is cooled down below the temperature of the air.  It differs in quantity with the radiating or cooling surface; that which has absorbed and retained the most heat during the day radiates the most at night and furnishes the most cold in return.

Hoar-frost, such as we find on our window-panes, or on the grass, is the moisture of the warm air cooled down and frozen, and is produced when the cold at the surface is below the freezing-point.  What we in common parlance call the action of frost, and which in this climate is well known to be very powerful, is not particularly injurious to organized bodies.