320 Description of a new portable Barometer, [APRIt, 
nient height in the tube, when the bladder is immersed in water at 
the temperature of 60° Fahr., which is the point of extreme mois- 
ture. Extreme dryness is obtained by inclosing the bulb in a glass 
vessel containing a portion of sulphuric acid of the specific gravity 
1:850. This range is divided into 100 equal parts, if the bore of 
the tube be equal, commencing at extreme moisture; and is so 
graduated on the scale to which the instrument is attached. 
. Dr. Thomson has suggested in his Annals that it would be better 
to reverse the scale by placing 0 at the point of extreme dryness, 
and 100° at that of extreme moisture. Whatever comes from so 
eminent a chemist is entitled to the fullest consideration. My 
principal reason for making zero at extreme moisture is, that it is a 
more invariable point than extreme dryness, and therefore, perhaps, 
a more certain foundation for calculation. ‘The point of extreme 
dryness will vary with the specific gravity of the sulphuric acid 
which is used, as well as with temperature ; but extreme moisture, 
by depending on water alone, is subject to no variation on that ac- 
count, and the temperature is in this operation more readily main- 
tained at the desired point. I acknowledge that, on the first view 
of the case, it appears proper that the numbers on the scale should 
decrease with a corresponding decrease of moisture ; but it is also 
to be considered that the dryness produced by sulphuric acid is by 
no means the absence of all moisture, but merely a relative term. 
With more powerful absorbents mercury may be raised higher; on 
the contrary, immersion in water is absolute dampness, and no 
means which I have discovered can make the mercury sink lower. 
The graduation of the instrument having in this manner been 
completed, a porous piece of wood is inserted in the end of the 
tube, and it is terminated by a small brass cap. The correction for 
temperature is ascertained either by obtaining the ratio between the 
eapacity of the bladder and the bore of the tube, which, together 
with the rate of expansion of mercury, will give the length of a 
thermometrical degree ; or, by immersing the instrument in water 
at different temperatures, and marking the rise of the mercury. 
This correction will vary in every bygroieter, for the same reason 
that causes a difference in the length of thermometrical scales. 
The hygrometer in this state, with its bulb uncovered, answers 
very well asa domestic instrument; but when it is agitated the 
column of mercury is apt to burst the bladder. In order to render 
it portable the bulb is inserted through the top of a cistern of box- 
wood incased with brass. ‘This cistern has its bottom composed of 
a leather bag, which is acted on by ascrew, and is likewise incased 
by a brass tube. 
When the instrument is to be packed for carriage, the cistern is 
rather more than half filled with mercury, and screwed to its cap, 
through which the bladder is inserted. By screwing up the leather 
bottom of the cistern, the air within it will be displaced through 
the pores of the box-wood, and the mercury will wholly encompass 
