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VI. On the Relation of the Air and Evaporation Temperatures to the Temperature of 
the Dew-Point, as determined hy Mr. Glaisher’s Hygrometrical Tables, founded 
on the Factors deduced from the six-hourly observations made at the Royal Obser- 
vatory, Greenwich. By John Fletcher Miller, F.R.S., F.R.A.S., Assoc. Inst. 
C.E. 8^c. Communicated by Lieut. -Col. Sabine, For. Sec. R.S. 
Received March 14, — Read May 30, 1850. 
Remarks. 
Of all the instruments observed and recorded by the meteorologist, the hygrometer 
is in many respects the most valuable, both in a scientific and practical point of view ; 
for, by a simple knowledge of the point at which the atmosphere can contain no more 
vapour without precipitation at its existing temperature, and which is called the 
point of saturation or temperature of the dew-point, many other highly important 
collateral conditions, such as the elastic force of vapour, the weight of vapour in a 
cubic foot of air, and the additional weight required for complete saturation at all 
temperatures of the air, the relative degree of humidity, and the weight of a cubic 
foot of air under the ever-varying circumstances of heat, moisture and pressure, may 
readily be obtained, either by well-known formulae, or by inspection from tables pre- 
pared for the purpose. But the hygrometer is also of no small value in the arts and 
in the practical concerns of life ; as a prognosticator of the weather it stands second 
to no other instrument except the barometer, which it often rivals in the faithfulness 
of its movements; whilst the condition of the air as to moisture or dryness is ac- 
knowledged by all physicians to have no slight effect on the human body, both in its 
normal and abnormal state; but more especially are its influences visible on the frame 
of the invalid, either in aiding to restore healthy functions, or in promoting the ex- 
acerbation of disease. 
It is therefore of no small moment that scientific observers, at least, should possess 
an instrument by which that important element, the dew-point, can be accurately and 
expeditiously obtained. 
Dalton and other old meteorologists made use of glass vessels filled with spring 
water, which retains a nearly uniform temperature throughout the year; and, in the 
summer months, it was generally found sufficient to cause a deposit of moisture on 
the outside surface of the glass. In winter, ice or snow was mixed with the water, 
or when the temperature was very low, freezing mixtures were employed. 
The dew was carefully wiped from the surface of the vessel as long as it continued 
