On Thjgrornetry. 
190 
With regard to the invisible aqueous j;as which we may call vapour, we have ascer- 
tained the following facts : 1. Like air its rarity increases and consequently its 
speri ‘c gravity decrea es with the temperature: the law too is the same which 
air follows. 2.’ Like air it has a certain spring or elasticity which, like Jhat of an, is 
capable of be'rg ensured by the column of mercury it will support. 3*. Unlike 
air its compressibility is limited; for on exceeding a certain pressure, which is differ- 
ent at different temperatures, the vapour changes or condenses into cloud. 
Schol. The pre sure which aqueous vapour will bear without condensation is 
called the maximum tension or constituent force of the vapour ; sometimes simply 
the force or tension. It is generally expressed in inches of mercury. Thus at 
temperature 212, it is 30 inches ; at 192*, 20 inches ; at I62j # , 10 inches ; at 135% 
5 inches ; and at 8 1 inch. These temperatures are called the constituent tem- 
peratures of vapour having those tensions. 
4. With air, we know, cold has the same effect as compression : so with aqueous 
vapour. If we cool a portion of the latter below the constituent temperature, that is 
to say, the tempteraure answering to its tension or elastic force ; part of it will be 
changed or condensed into cloud. 
Schol. Vapour may he heated above its constituent temperature, but cannot be 
cooled below it without change of state. It is the lowest temperature at which vapour 
of that tension can exist. It is therefore called constituent , inasmuch as the vapour, if 
cooled below it, changes its state or constitution from the gaseous to the fluid. 
5. Two volumes of air of different temperatures being mixed, the resulting tem- 
perature will be the arithmetical mean : two portions of aqueous vapour of different 
constituent temperatures being mixed, the resulting temperature will be something 
higher. The constituent tension of vapour in fact increases in a higher ratio as the 
temperature is higher : the mean of the constituent tensions can therefore never cor- 
respond with the mean of the temperatures answering thereto ; but is always some- 
thing more- In order, then, to re-establish this correspondence, part of the vapour 
suffers condensation ; by which the temperature being raised, and the tension dimi- 
nished, the two powers are found at last in equilibrio. An example will make this 
clearer. The constituent temperature of vapour of 1 inch force is 8»< ; of ,5 inch ,59*.— 
The mean of the temperatures is 693, and of the tensions ,75 inch. But a tempera- 
ture of 695 , can only uphold vapour of tension ,71 inch ; vapour of a greater tension 
as of ,75 in. on being exposed to a temperature of 69 , 5 would he partially condensed; 
till the extrication of heat and diminution of tension together, restored the relation 
that must exist between the two phenomena. In fact vapour of tension, 75 inch 
would require a temperature of 71. The higher the temperatures, the greater will 
the discordance be. 
Schol. The preceding fact furnished the grounds of Dr. Hutton’s theory of 
rain. Two portions of vapour, each at its lowest temperature, being brought into in- 
timate mixture, the. result is partial condensation, which, if it be sufficiently copious 
to allow the small particles to congregate into drops of sufficient size, will ter- 
minate in rain; but if the supply of moisture fail, and the particles remain 
small as to be buoyed up by the resistance of the air, then is a cloud or fog the 
consequence. 
It appears then that the great point in metereology is to ascertain the constituent 
temperature of the vapour actually existing in the air. Comparing this temperature 
with that likely to be established by currents or radiation, we obtain a measure of 
the probability of rain ; which ought thus to be more or less copious, as we find the 
temperatures more or less differ. The constituent temperature of vapour is also 
called the temperature of the dew point . It is determined directly by Dalton’s 
experiment described in my former paper, or by Mr. Daniell’s hygrometer founded 
on the same principle. 
In India it does not appear that these methods are at all times practicable, so 
low is the dew point occasionally. Under the most favourable circumstances they 
are troublesome, and not likely to be repeated too frequently. 
The moistened-bulb thermometer which indicates the temperature of an evapo- 
rating surface, affords much greater facilities ; facilities equal in every respect to 
those available in observing the temperature. A great point is gained too by using 
this instrument ; the power of observing the extremes, which no hygrometer yet 
proposed has effected. By employing a register thermometer, and covering the 
bulbs with cloth kept moistened, we may obtain an idea of the maximum and mini- 
* This must be understood to refer to ordinary pressures. By employing ex® 
traordinary force it is supposed that air also may be liquefied. 
