GLEANINGS 
IN 
SCIENCE. 
./Vo. 7. -July, 1829. 
I. — On Hygrometry. — No. 2 . 
To the Editor of “ Gleanings in Science.” 
Sir, 
In my communication published in your second number, I promised to return to 
the subject of hygrometry, and show what are yet the desiderata of the question. I 
mentioned, that we laboured chiefly under the want of accurate and full experiments, 
to enable us to determine the constants of the equation, (which I there gave,) in order 
to apply it to practice. I have just received your third number, in which has appeared 
another communication on the same subject by your correspondent P. This paper 
supplies the very experiments I was in search of ; and although not quite prepared to 
follow up the subject with the fulness I contemplated, I cannot deny myself the pleasure 
of showing the utility of my formula, by applying it to these admirable experiments. 
Before I conclude my letter allow me to return this gentleman tuy thanks for his valu- 
able communication, which has undoubtedly finished the experimental part of the edi- 
fice commenced by Dalton. Unless I am much mistaken, there remains little now to 
be done in this branch of physics; and the problem of determining the moisture in 
the air is become as simple nearly and as level to ordinary' capacities as that of the 
temperature or the density. It is a curious illustration of the subject to think that 
metercologists have so long had in their hands, without knowing it, the best hygro- 
meter that can be devised; the most accurate and the cheapest ; while the inventive 
spirit has sought it in vain in oat beards, deal boards, catgut, whalebone, ivory tubes, 
rat’s bladders, and the thousand and one other substances proposed from time to 
time for this purpose. But I am reminded of the narrow limits of your work, and 
must therefore hasten to the proper subject of my communication. With every 
wish for the success of it, 
I am, Mr. Editor, 
Your obedt. servt. 
D. 
From the remarks I have had the advantage of hearing on my first paper, pro- 
tected by my incognito, 1 incline to draw' the conclusion that I have myself fallen 
into the mistake censured by me in another “ of having perplexed plain men and 
obscured a simple suhject.” 1 am sensible, in fact, that I have given my reader* 
credit for being equally familiar with the subject as I was myself. I have in con- 
sequence slurred over many subjects that required explanation ; and have thus be- 
come obscure where I could least afford it : I shall endeavour to avoid this error on 
the present occasion. 
The object of hygrometry is to determine the quantity of moisture existing at any 
moment in any gas, but principally in the atmosphere ; and the probability of rain. 
The moisture in the air exists in two states or conditions : l. That of a trans- 
parent invisible gas ; and 2. That of cloud. In the first case it has all the proper- 
ties of the gases ; such as elasticity, expansibility by r beat, compressibility by pres- 
sure, &c. In the state of cloud it is otherwise : it has changed the gaseous for 
the fluid form ; but the particles into which the gaseous atoms have condensed, are so 
wide apart, as to remain so, notbeing attracted by each other ; and they are so small, 
that they are borne up by the resistance of the air, and thus float in a medium the 
specific gravity of which is less than l-800th of their own*. 
* The resistance of the air to any body increases or decreases as the square, the 
weight as the cube, of the diameter. We may imagine then a size of particle of 
the densest matter which shall float in the lightest medium. 
