i817.) : and a new Hygrometer. 317 
In climates where the temperature of the air is above 75° Fahr. 
opposite means must of course be employed to cool the cistern, and 
the observation be made on its rising to the proper degree. In such 
a case the correction will be more simple. At all events, the ob- 
server has it in his power to choose either means. 
Before the brass tube is turned back to cover the slit, the baro-. 
meter is held in a slanting position with its cistern uppermost, so 
that the mercury may descend slowly in the tube, and force the air 
before it through the porous piece of wood. When all the air is 
expelled, and the mercury rests on the wood, the stop-cock is shut, 
in order to prevent the entrance of air, which might tend to sepa- 
rate the mercury in the tube, although it could not enter the cistern. 
By this means the instrument is rendered perfectly portable, and: 
not liable to be put out of order, however much it may be agitated. 
When carried, the cistern should be kept uppermost. ‘The size of 
the instrument is only about 12 inches long, and 14 inch in 
diameter, so that it can readily be put into the pocket. 
When it is not found convenient to bring the barometer to the 
temperature already specified before each observation, the degree of 
the fixed thermometer must also be read off at the different stations, 
and registered, and the value in parts of an inch of its deficiency of 
75° added to, or its surplus subtracted from, the barometrical quan- 
tity, according as it is above or below the standard temperature. 
It has already been noticed that the linear value of a specified, 
number of thermometrical degrees is always engraved on the scale, 
of each instrument, in order to facilitate this correction. 
When the instrument is made as a domestic or marine barometer, 
it is fitted with a sliding scale, on which the thermometrical degrees, 
are engraved. By inspecting the thermometer, the temperature is. 
to be ascertained, and the sliding scale is moved until the number 
on it which corresponds to the deficiency of temperature in the 
barometer from the proper standard is on a line with the mercurial, 
column. The top of the sliding scale is then pointing to the true, 
barometrical height. ‘This correction is simply adding the quantity 
which the mercurial column would acquire in height if raised to the 
standard temperature. By making this standard above the general 
heat of the atmosphere, the correction is always additive, and 
therefore easily performed, and not likely to create any confusion, , 
The advantages which this instrument possesses over the common 
barometer in the mensuration of heights will be immediately per- 
ceived. Its greater portability would be sufficient to entitle it to a 
preference, but it likewise promises to be more accurate in practice. 
It is well known that a considerable difference exists in the specific 
gravity of the mercury which is met with for sale ; and the makers 
of barometers have not the means, or could not bestow time, for 
purifying all that they use. It is, therefore, employed in the state 
rithmic tables the metrical value from 15 inches to 31 inches of the barometer in 
thousands of an inch; so that the approximate heigit in feet is at once obtained + 
by inspection, from the difference of the observations. The corrections for tem- 
perature are likewise easily performed, 
