28 
HINTS TO TRAVELLERS. 
Next look along the vernier until one of its lines is found to coincide with 
a line on the fixed scale. In B this will be found to be the case with 
the second line above the figure “ 2.” 
therefore 
The reading of the barometer is 
On fixed scale 
.. 29-750 
On vernier (12 x *002) 
•024 
Correct reading 
.. 29-774 
Should two lines on the vernier be in equally near agreement with two 
on the fixed scale, then the intermediate value should be adopted. 
5. Lower the mercury in the cistern by turning the screw at the bottom 
until the surface is well below the ivory point; this is done to prevent the 
collection of impurities. 
The transport of barometers requires very great care in order to 
prevent the introduction of air into the tube or the fracture of the tube 
by the impact of the mercury against the top. To reduce the risk of 
these accidents, the barometer must be carried with the tube quite full of 
mercury , and in an inverted position, at least with the cistern end kept 
higher than the top of the tube. The flexible cistern of the Fortin type 
of barometer allows of it being screwed up tight so as to fill the tube 
and close the lower end of it. In case of breakage, the operation of 
fitting a new tube is not very difficult, but unless the tube has been 
carried out ready filled with mercury, this cannot well be attempted. In 
order to drive out the film of air adhering to the glass on the inside, it is 
necessary, after filling the tube, to raise its temperature to the boiling- 
point of mercury. No one should attempt either to fill or to change a 
barometer tube unless he has had practice in doing so under expert 
supervision beforehand. 
The Collie Portable Mercurial Barometer .—This instrument is not 
likely to be broken in travelling. It is quickly set up, and from such 
tests as have been applied, it appears to give excellent results. The 
cistern and vacuum tube at the top are of equal diameter, and are 
connected by a flexible tube, and the difference in level of the mercury 
may be measured directly by means of a graduated rod, or as in Deasy’s 
mounting by means of a vernier. There is no attached thermometer, but 
if the instrument be used in the open air, and is exposed for ten minutes 
