The life and correspondence of Andrew Combe / By George Combe.

  • George Combe
Date:
1850
Catalogue details

Licence: Public Domain Mark

Credit: The life and correspondence of Andrew Combe / By George Combe. Source: Wellcome Collection.

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    “lhe conditions of the atmosphere which influence the abstraction of heat from the body are three, viz., its temperature, its motion, and its hygrometric state ; and the character of a climate mainly depends on the manner in which these are combined. “TI. When the temperature of the air is low, heat is rapidly ab- sorbed from the surface of the body, and it could not preserve the natural temperature, were radiation, and the free contact of the air, not prevented by thick clothing. “TJ. Air, of a given temperature, in a state of motion, deprives a warm body of a much greater quantity of heat than air of the same temperature at rest.” According to Sir John Leslie, air with a velocity of eight miles an hour would, in a given time, deprive a body of double the quantity of heat which the same air would do in a state of rest ; with a velocity of 16 miles an hour, three times the quantity ; with one of 24 miles, four times; and with one of 32 miles, five times. “III. The hygrometric state of the air is, next to its temperature, the most important condition which influences the abstraction of heat from the human body. ‘“‘ Tt is usual, in treating of aqueous vapour, to represent its elas- ticity by the column of mercury it could support; and as the absolute weight of vapour that could exist in a given space is always propor- tional to its elasticity, the quantity which could exist in the same space, at different temperatures, is usually indicated by the correspond- ing elasticities. The quantity of aqueous vapour which can exist in the atmosphere is very small at low temperatures, but it increases — with great rapidity as the temperature advances ; for at the tempera- ture of 16°°3 Fahrenheit, the elasticity is ‘100 inches, at 34°-0 it is 200 inches, at 52°°8 it is -400 inches, at 73°°0 it is °800, and at 94°°8 it is 1°600 inches, or it is doubled for every increase of 19°°6. The air is seldom or never completely saturated with moisture, and would not begin to deposit it until cooled down more or less; the temperature at which the deposition commences is called the dew-point. When air is not fully saturated with vapour, it is always capable of taking up an additional quantity, which is proportional to the difference be- tween the elasticity of vapour at the dew-point, and at the tempera- ture of the air: thus, were the temperature of the air 30°°0, while the dew-point was 16°3, the additional quantity of vapour the air would be capable of taking up would be represented by ‘100; and, were the temperature of the air 73°0, and the dew-point 52°°8, it would be represented by -400.” “The quantity of heat necessary for the formation of vapour is the same, whatever may be the sensible temperature of the vapour, and, of course, the quantity necessary for converting any portion of water
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