$i8 Description of a new portable Barometer,  ([Apnit, 
which they buy it; and the barometer is graduated without reference 
to the gravity of the mercury which it contains. A constant source 
of error arises from this circumstance, which will be avoided in the 
instrument that I have described; as it will be only necessary to 
ascertain with accuracy the specific gravity of the mercury with 
which the standard barometer is filled. Both extremes of the new 
instrument being derived from it, a compensation will be effected 
for any difference in the gravity of the mercury which is employed. 
In the common barometer another source of error arises from the 
difficulty of ascertaining the temperature of its column of mercury, 
which may be considerably heated by holding it in the hand without 
perceptibly effecting the thermometer placed in the mounting. In 
fact, this correction is too often omitted entirely, from the uncertain 
way in which it can be obtained. From the thermometer being 
included within the cistern of the new instrument, the real tem- 
perature will always be indicated ; and as it is graduated at a fixed 
point, and a correction made for any variation of temperature in the 
standard barometer, this source of inaccuracy will be entirely re- 
‘mnoved, ; 
The principle upon which this instrument is constructed may be 
yet further extended; and, by substituting lighter fluids for the 
mercury, barometers of such delicacy may be formed as to be prac- 
tically employed with advantage in levelling. And if it be found, 
on experience, that the confined atmospherical air loses a portion of 
exygen by acting on the mercury, hydrogen or nitrogen may be 
employed, which will undergo no change. 
In atmospherical phenomena, the instrument next in importance 
to the thermometer and barometer is, perhaps, the one which ascer- 
tains variations in its degree of moisture. Within these few years 
much progress has been made in this department of meteorology, 
which, in its relation to the comforts and welfare of mankind, is 
confessedly of the highest importance. Many eminent men have 
directed their labours to its advancement, and its instruments have 
consequently been multiplied and improved. But one which is 
with regard to moisture what the thermometer is to heat, still re- 
mains a desideratum in the science. 
A great variety of hygrometers have been at different times con- 
structed by Smeaton, Saussure, Deluc, and other scientific men ; 
and more lately Professor Leslie and Captain Kater have invented 
very excellent instruments of this kind, which have given much 
insight into the phenomena of the weather. It is well known that 
Professor Leslie has for some years devoted himself to this study 
with the most eminent success. 
The principles upon which hygrometers have been constructed 
are, the absorption of heat by evaporation, the changes which take 
place in the length of animal and vegetable fibres by alterations in 
their degree of moisture, the variations in the capacity of a hollow 
ball of ivory from the same cause, and likewise the changes in 
weight of deliquescent bodies. 
The objections to all hygrometers that have yet been invented 
