Aids to the mathematics of hygiene / [Robert Bruce Ferguson].

  • Ferguson, R. Bruce (Robert Bruce)
Date:
1904
Catalogue details

Licence: In copyright

Credit: Aids to the mathematics of hygiene / [Robert Bruce Ferguson]. Source: Wellcome Collection.

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    If the air be saturated no evaporation is possible, and the two thermometers will read alike; if not saturated, the wet-bulb will read lower than the dry- bulb, but not so low as the dew-point; in fact, the temperature of the wet-bulb is always above the dew- point. When the dry-bulb stands at 530 F. the dew- point is as much below the wet-bulb as the wet-bulb is below the dry-bulb. Above this temperature, the wet-bulb approaches nearer the dew-point, and the reverse is the case below that temperature. Glaisher has empirically compiled some tables, whereby the difference between the dew-point and the wet-bulb bears a constant ratio to the difference between the wet-bulb and the dry bulb ; so that, if the reading of the dry-bulb be given, the dew-point can be calculated. According to him, the temperature of the dew-point is obtained by multiplying the difference between the wet-and-dry-bulb temperatures by a constant factor (‘ Glaisher’s factor’), and subtracting the product thus obtained from the dry-bulb temperature, thus : Dew-point=Td—[(Td —Tw) x F]. Where Td = temperature of dry-bulb (Fahrenheit), Tw = temperature of wet-bulb „ F = factor (found opposite the dry-bulb temperature in the table). If Td = 6o° F. and Tw=54° F., then (from table) F= i'88 and dew-point=6o —[(60 - 54) x r88] = 48'72° F. From this formula Glaisher calculated his tables, which give the dew-point on inspection. The dew-point may also be found by Apjohn s formula. For this purpose we require a table of vapour-tensions (page 93).
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