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LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
the bottle of wine or beer corresponding to the bulb of the thermometer ; the straw 
lying thinly over it represents the muslin, and the operations of the watering-pot com- 
plete the wet bulb apparatus, evaporation does the rest ; but as the velocity and dryness 
of the wind regulate this, it is plain that such an instrument can only give uncertain 
and fallacious results when used to determine, with any pretension to accuracy, the 
fractional saturation of the atmosphere. I come now to a source of error in my 
reductions of the wet bulb observations which I have collected in this paper from 
various parts in India, a source of error that may operate with a greater or less 
power as the depression of the wet bulb is greater or less. I allude to the formula 
used for the reductions of the readings of the wet bulb. M. Regnault says that 
M. Gay-Lussac was the first to propose the determination of the dew-point by the 
observations of a dry and wet bulb apparatus*, but that to effect the object satisfac- 
torily it would require extensive observations upon which to found tables. Subse- 
quently to the period of M. Gay-Lussac’s proposition, August, Professor at Berlin, 
occupied himself with the subject and published some papers, in which he sought 
to determine, upon theoretical considerations, the formula by which the elastic 
force of aqueous vapour, really existing in the air, could be found by the difference 
of temperature of a dry and wet bulb thermometer. The dry and wet bulb apparatus 
he called a Psychrometre. His chief memoir is published in the Annalen der Physik 
und Chemie, V. Band. Leipzig, 1825. It will suffice to say that he considered the 
wet bulb surrounded at all times with a coat of vapour of the same temperature as 
the bulb, which was, he stated, necessarily lower than that of the surrounding air, — 
that the successive supplies of air coming into contact with the wet bulb, parted with 
a portion of their heat and took the temperature of the wet bulb ; but on the other 
hand, the air so supplied in vaporizing the water upon the surface of the wet bulb 
took from it a portion of its heat ; and a stationary state of the temperature of the 
wet bulb was established by these two quantities of heat balancing each other. 
August’s formula was f h, where t denotes the tempe- 
rature of the dry bulb (in Centigrade degrees), t' the temperature of the wet bulb, 
y the specific heat of dry air, § the density of aqueous vapour, X the latent heat 
of aqueous vapour between the temperatures t and t' , a the specific heat of vapour, 
h the height of the barometer, f the elastic force of vapour in saturated air at the 
temperature t' , and x the elastic force of the vapour actually existing in the atmo- 
sphere ; x,f and h being expressed in inches, or in terms of any common unit. 
the wet straw process, on the 21st of May 1850, and preceding days. Temperature of air in shade, free from 
radiation, 98° Fahr. Temperature of water in bottles under wet straw exposed to wind 65°, difference .33°. 
The dew-point by Apjohn’s formula would be about 41°’ 7, and by Glaisher’s factors about 48°’5. My friend 
says, “ When the wind does not blow, the temperature of the water in the bottles, under the straw, cannot be 
got lower than 71° Fahr. When the wind blows, the bottles cool to 65° Fahr.” 
* Annales de Chimie et de Physique, 2nd series, t. xxi. p. 91. 
