1833.] and Observations on Wet Barometers. 261 
mn Cie htha se bila lislbe sales ieidls Wl oie salaiade oe ees ale alee —.687 
eT eho ee) Roepe ys ees) Pecans ee ae ae —.702 
It is needless to say, that in all these cases every care’ was taken to 
exclude air. It appears, therefore, that after filling two or three times, 
the mercurial column stood nearly .70 too low. The hygrometer at 
the same shewed that the aqueous tension was .60, which so nearly 
agreed with the former (making allowance for capillary action), that 
we had no doubt at the time that the depression was caused by mois- 
ture, attracted by the tube from the air, the frequent renewal of which 
allowed the surface of the glass to attain a state of hygrometric equi- 
librium with the latter. That such was the case was further proved 
by repeating the operation on a subsequent day, when the air was much 
drier ; the hygrometer then shewing a tension of .45, the barometer stood 
between .394 and .415 lower than the standard. 
It is well known how pertinaciously water adheres to the surface of 
glass: in damp weather an electrical machine cannot be worked unless 
dried by heat ; and any glass tube, even in dry weather, if heated in one 
part, will shew the presence of water by a condensation of minute glo- 
bules in the colder parts of the tube. Such facts, in conjunction with 
the positive testimony now adduced, prove that implicit reliance should 
not be placed upon this kind of ‘barometer, especially for the measure- 
ment of heights. The different quality of the glass may also have great 
influence on the aqueous action, according to the predominance or 
otherwise of alkali in its composition. 
Mr. Farapay has recommended that borax should be substituted for 
alkali inthe composition of glass for astronomical purposes, on account | 
of the liability of the alkaline glasses to injury by the wet, but we have 
not yet seen any notice of the result of such trial on a large scale. 
Perhaps the barometrical effect now noticed might be turned to advan- 
tage, as a mode of measuring the hygrometrical quality of glasses of 
different founts. Of five dry tubes of the same bore, filled together 
and placed upright in the same reservoir of mercury, no two were found 
to agree together, whereas upon wetting the interior of the same 
tubes, they then agreed very well together, and (after making correction 
for the aqueous tension corresponding to the temperature of the mercury) 
also very nearly coincided with the standard barometer, without any 
allowance for capillarity. 
This circumstance suggested an easy and certain method of turning 
the tubes to account in the survey, namely, to use them always wet and 
make the necessary addition. The most defective and dirty tube 
might in this way be rendered as serviceable as the cleanest, and I 
