aO4 
fore present our readers with the gene- 
ral results in Dr. Herschel’s own words. 
és In order to see well with telescopes, 
Wt is required that the temperature of the at- 
mosphere and mirror should be uniform, and 
the air fraught with moisture.” 
«This being admitted, we shall find no 
difficulty in accounting for every one of the 
foregoing observations. 
*« Ifan uniform temperature be necessary, a 
frost afier mild weather, or a thaw after frost, 
will derange the performance of our mirrors, 
till either the frost or the mild weather are 
sufficiently settled, that the temperature of 
the mirror may accommodate itself to that of 
the air. For, till such an uniformity with 
the open air, in the temperature of the mir- 
ror, the tube, the eye-glasses, and I would 
_ almost add the observer, be obtained, we can- 
not expect to see well. 
** But, when a frost, though very severe, 
becomes settled, the mirror will soon accom- 
miodate itself to the temperature; and we 
shall find our telescopes to act well. 
«© This explains, with equal facility, why 
no telescope just brought out of a warm 
room cau act properly. | 
«* Nor can we ever expect to make a deli- 
cate observation with high magnifying 
powers, when looking through a door, win- 
dow, or slit in the roof of an observatory ; 
even a confined place, though in the open air, 
will be detrimental. 
«© Tt equally shows, that windy weather 
in general, which nmlust occasion a mixture 
of airs of different, temperatures, cannot be 
favourable to distinct vision. 
«* The same remark will apply to aurore 
boreales, when they induce, as they often do, 
a considerable change in the temperature of 
the different regions of the air. 
«« But should they not be accompanied by 
such a change, there seems to be no reason 
why they should injure vision. 
«* The warm exhalations from the roof of 
a house in a cold night must disturb the uni- 
formity of the temperature of a small portion 
of air; so that stars which are over the house, 
and at no considerable distance, may be.af- 
fected by it. 
«« Sometimes the weather appears to be fine, 
and yet our telescopes will not act well. This 
may be owing to dryness occasioned by an 
easterly wind ; or to a change of temperature, 
arising from an agitation of the upper regions 
of the atmosphere. 
«“ Or, possibly, to both these causes com- 
bined together. If moisture in the atmo- 
sphere be necessary, dry air cannot be proper 
for vision. : 
‘« And. therefore, on the contrary, damp- 
ness and haziness of the atmosphere must be 
favourable to vision. Fogs also, which cer- 
tainly denote abundance of moisture, must 
be very favourable to distinet vision. Nay, 
if the observatory should be surrounded by 
water, we need be under no apprehension on 
GENERAL SCIENCE. ra 
that account. Perhaps, were we to erect # 
building for astronomical purposes only, we 
ought not to object to grounds which are oc- 
castonally flooded ; the neighbourhood of a 
river, a lake, or other generally called damp 
situations.” 
9. dn Account of some Experiments and Ol- 
servations on the Constituent Parts of cer- 
tain astringent Vegetables, and on their 
Operation in Tanning. ~ By H. Davy, 
F:sq. Professor of Chemistry in the Royal 
Institution. . 
Mr. Davy is well known to the phi- 
losophical world as an able and acute in- 
vestigator of chemical phenomena, and 
we will venture to assure our readers 
that they will find the present memoir 
to be worthy of the high reputation of 
its author. The first part contains some 
preliminary observations on the analysis 
of astringent vegetable infusions. For 
ascertaining the presence and quantity 
of tannin, Mr. Davy makes use of a 
solution of isinglass in distilled water ; 
the application, however, of this re- 
agent, he justly observes, requires some 
caution, since a concentrated solution of 
gelatin affords a considerably larger 
precipitate with a given quantity of tan- 
nin than when it is diluted. It is highly 
requisite also, that the gelatin should 
be quite fresh, as a very slight chemical 
change in it will, for the most part, de- 
stroy its power of precipitating’ with 
tannin. 
After the separation of tannin from 
an astringent infusion, the next process 
is to disengage the gallic acid from the 
extractive matter; this, however, can- 
not be done with perfect accuracy; Mr, 
Davy appears to prefer the muriat of 
alumine, which throws down the greater 
part of the extractive matter, without 
materially acting on the gallic acid. 
The infusion of galls is the first astrin- 
gent liquor treated of. 
The greatest sp. gravity of this, when- 
prepared with the best Aleppo galls, at 
55° Fahrenheit is = 1.068. . 400 grains 
yield by evaporation 53 grains of solid | 
matter, consisting of 9-1Uths tannin, and 
1-10th gallic acid, with a little extractive 
matter. Sulphuric and muriatic acids 
occasion a white precipitate of tannin, 
with some gallic acid and extract, which 
is resoluble in warm water. Nitric 
acid, except very weak, destroys both 
the tannin and gallic acid. Potash 
suspends the action of gelatin on the in- 
fusion, but its effect may be counteracted 
by the addition of an acid. The alka- 
