34 Revieivs and Extracts. 
observations, made at ten A.M. and ten P.M. in 1827, was 21615,41 and this 
numbei- divided by the number of observations, 730, or twice the number of 
days in that year gives 29,61 inches as the height or changeable point of the 
Barometer. It is from the mercury's being above or below this point then, 
that you are to conchide what weather may be expected from its indications 
or changes, From a want of this knowledge, many are led into a mistake by 
supposing that the words Fair, Change, and Kain, engraved on the scale of 
the Barometer indicate such weather in all places, whereas, they can only do 
so at one particular altitude." 
Having now explained the principal causes of failure in using the common 
Barometer for general purposes, he next proceeds to give a description of 
that Instrument, to be used where the greatest accuracy is required, for the 
determination of minute changes of the height of the ground above the sea. 
" The instrument to be used for such purposes should have a tube of large 
diameter, in which the mercury is boiled, to expel from it all air or moisture. 
In the cistern containing the mercury there is another tube of the same inter- 
nal diameter as that of the Barometer, with a metal ring embracing it, and an 
adjustment is made at each observation, by forcing the mercury from the cis- 
tern by means of a screw, till its surface cuts off the light seen through the 
glass tube, below the ring. An instrument of this construction is said to 
have an adjustable surface ; — that is, the surface of the mercury in the cistern 
is always brought to a fixed point, consequently the length of the mercurial 
column is at ail times observed above the point. For the measurement of 
heights, the scale of this Instrument is made to read off to the 1000th part of an 
inch. There has lately been constructed a cast-iron cistern, which renders 
the Instrument much less liable to injury from carriage, and gives great sta- 
bity to the mercury during observation. The method of using the Barometer 
for the purpose, is to observe its height at the upper and lower stations ; and 
the temperature of the mercury in the barometer is to be ascertained by means 
of a tliermometer attached to it, and also that of the air, by one freely exposed. 
All the instruments must be well defended from the sunshine, and when time 
will permit, it is of great importance to allow them to remain till both the 
attaclied and detached thermometers indicate the same temperature. When 
two Barometers are used, that at the lower station should be observed at the 
same moment as that at the upper, by signals, or at times previously fi.xed 
upon by the observers. When one instrument only is used, any change in 
the Barometer that may take place during the timo required in passing from 
one station to another, gives an error in the deduced height. Steady weather 
should always be selected, and though great distances may intervene, between 
the situations, the results may be equally accurate. In a series of observations 
in which 1 was engaged to determine the height of Ben Lomond, where four 
observers were engaged, one at the top of the mountain, a second at the base, 
and the other two at Edinburgh, a distance of about 00 miles ; after comparing 
the heights deduced from the three lower of these observations, the agreement 
was found to be very close." He next proceeds "to give a table showing the 
elevation corresponding to the depression of the 10th part of an inch on 
the Barometer, when at its mean state, by an inspection of which an idea of the 
altitude may be formed. Thus, if the changeable point of the Barometer, in 
any situation, be 29,4, and that at the level of the sea 29,9, subtracting the 
height corresponding to this in the table is 439 feet. This will be nearly the 
alliiiida of the situation. 
