26 
HINTS TO TRAVELLERS. 
is likely to be much more serviceable to the ordinary traveller on the 
march than a standard mercurial barometer, every packing and un¬ 
packing of which exposes it to the risk of breakage, or to the equally 
fatal risk of air obtaining access to the vacuous space at the top of the 
tube. 
We shall describe two types of mercurial barometer, one the Fortin, 
which is best adapted for use at a fixed station, the other devised by 
Prof. Collie and Capt. Deasy, which is portable enough for the use of 
travellers. 
The Fortin Barometer .—The barometer must be kept in a room with 
as equable a temperature as possible; the instrument must be abso¬ 
lutely vertical—hence it should be hung freely and not touched while 
it is being read; it must be in a good light, and yet be sheltered from the 
direct rays of the sun. The measurement of the height of any mercurial 
barometer is that of the difference of level between the surface of the 
mercury in the tube and the surface of the mercury in the cistern. When 
the mercury rises in the tube it falls in the cistern, and vice versa , 
although when the cistern is much wider than the tube the changes of 
level there are much less than those in the tube. In most barometers an 
arbitrary correction is made to allow for this change, the “ inches ” en¬ 
graved on the scale not being true inches. In the Fortin barometer, 
however, the lower end of the measuring rod is brought in contact 
with the mercury in the cistern before every reading, and then the 
scale of inches engraved on the upper part of the measuring rod gives 
the true height of the column of mercury. In calculating the barometric 
pressure for purposes of comparison, five corrections have to be applied: 
(1) for temperature, which requires the temperature of the barometer at 
the time of reading to be observed, (2) for altitude, which necessitates 
knowing the elevation of the place of observation above sea-level, (3) for 
the force of gravity at sea-level, which requires the latitude to be known, 
(4) for the capillary attraction between the mercury and the glass tube, 
which is a constant for each barometer, (5) for the slight imperfection in 
engraving the scale (index error), which is also a constant for each 
instrument. 
It is enough for the observer at a fixed station, and to such alone can 
the use of a Fortin barometer be recommended, to read the temperature 
on the thermometer attached to the barometer and to read the height of 
