686 
GALBRAITH’S ASTRONOMICAL OBSERVATIONS. 
From this last formula, a scale may he rea- 
dily adapted to a level. 
2. Let the length L of one of the divisions 
of the scale of a level be oue-twentietii of aa 
inch, the run of the bubble two indies, the 
distance d one inch, and a fifth, and D five 
hundred feet, required a the value of one 
division of the scale in seconds ? 
R'" X L X d 
By formula (3) a" — _ 
D X r 
206264”'8x 0-05 X 12 
=1 ’*103 
6000 X 2 
3. At the distance D = 90‘6 feet, the vane 
of a levelling rod passed over 0-06 of a foot 
in a run of 25 divisions of the level, what was 
tlie value a ' of one division of the scale, and 
the radius of curvatuie of the level, L being 
one-lenth of an inch ? 
R’ X d 
By formula (4) a ' 
206264"*8 X 0-06 X n 
— =5"*5 
90 6 X 25 
By formula(5) p = 
R'X L 
a" 
R* X T*(> ^ T 5 
5'.5 5-5 X lOX 12 
206264-8 
=312 5 feet. 
C6o 
This result 5"-5 is nearly the value of one 
division of a level attached to a six-inch 
travelling circle ol Captain Kater’s consii uc- 
lion, made by Robinson. It is obvious, that 
the same method may be applied to determine 
the value of the divisions of a level belonging 
to larger instruments when required, and it is 
susceptible of very considerable accuracy 
when sufficient care is taken in performing the 
necessary operations. 
II. After having determined the value of 
the divisions of the scale of a level, it is next 
proper to adopt a simple and ready method 
of applying its effects to observations. 
Let e be the eye end of the telescope next 
the observer, o the object end, a' the value of 
one division of the level in seconds, n the 
numberof observations, and I theii effect when 
applied to the zenith distance. 
(e — o)a- 
l=- ( 6 ) 
2 n 
The sign must be changed when applied to 
the altitude. 
III. When three or more verniers are ap- 
plied to a circle, and the observations are re- 
peated and read each time, the mean result 
will be readily determined by the following 
formula in which X ^ ‘s the sum of the read- 
ings of all the verniers or microscopes, n the 
numberof observations,?; the number of ver- 
niers, and m the mean value of the whole. 
n V 
These formuise will apply with ease and 
ceitainiy to any case likely to occur in prac- 
tice. and are more simple than any I have 
seen. 
ly. The case to which they are now to be 
applied is one of a series of otrservations made 
by a small circle of Captain Kater’s construc- 
tion, to determine theobliquiiy of the ecliptic 
at the laie summer solstice, at Edinburgh* in 
latitude 55® 57’ 15” 67 N. 
It may seem to bean attempt much beyond 
the poweis qt so small an instrument, one of 
six inches diamelei, fu-nished with three 
verniers, each showing 15 and a level, indi- 
catinir: by each division only to tlie accuracy 
of 5 "5. \ el, the conectnessof the final result, 
w'hu'li differs from Bessel’s by about 1 §, and 
from mine, qi'tained Iry a comparison of the 
late obseivaiion^ made at Greenwich, with 
those ol Fiiadley, reduced with the best tables 
by I §, shows hovv much may be accom- 
plisireo with moderate means. With what 
pleasure would modern astronomers have con- 
templated the observations of Hipparchus 
and Ptolemy had they been made with such 
precision ! 
'i'o determine the obliquity of the ecliptic 
intlren.ost accutaie manner, the sun’s de- 
clination (daily if possible), near the solstices, 
must, it is well known, lie observed carefully 
for some time, and tlie results, by means of 
appropiiate fortnulre or tables, are_ieduced 
correctly to the moment of the solstice com- 
puted trom the best solar taldes, or obtained 
from corresponding observations. 
(To be continued.) 
EXPERIMENTS ON THE ABSORP- 
TION OF AIR BY WATER. 
By Thomas '1 hobtso.v, M. D., F. R. S. 
L. AND E., &c., 
Regius Professor of Chemistry in the Univer- 
sity of Glasgou). 
Not being aware of any direct experiments 
upon the sulijects mentioned in the litle of 
this paper, 1 amused myself, during the ear- 
ly part of the present summer, in making a 
few trials to satisfy myself whether the opi- 
nions at present entertained on these sub- 
jects were enlilied to corffiJence. I shall 
state my experiments on each of the different 
sui'jecfs in order. 
I.-OF THE QUANTITY OF AIR 
CONTAINED IN CLYDE WATER. 
The city of Glasgow is supplied with water 
pumped out of the river Clyde, and convey- 
ed to resei voirs in the higher parts of the 
town, from which it is conveyed in pipes to 
every house. As one of these pipes supplies 
my laboratory, I have only to turn a stock 
cock to obtain as much river water as I 
have occasion for. 
1. 1 filled a retort, the belly of which held 
168 cubic inches, and its throat and beak 75 
