14 
HINTS TO TRAVELLERS. 
of spirit, as it is a common fault to make it too small, and the tube 
carrying the wick should be long to prevent overheating the spirit. A 
screen, which may be made of tin to fold up, is most useful to place on 
the windward side, and at a very low temperature is almost indis¬ 
pensable, as the heat is otherwise carried off too rapidly for the water to 
boil properly. 
The Aneroid. 
The general appearance of the aneroid, of usual construction, is so 
well known that it requires no special description; it is an excellent 
instrument for laying down contour lines ; but for absolute heights it 
should be checked by the boiling-point thermometer, because its index 
error is apt to change; when thus checked it is a valuable instrument 
for measuring heights up to 8000 feet, but at greater elevations it is 
unreliable. It should be sent to the National Physical Laboratory to be 
tested, and have its errors determined before and after it has been used 
by a traveller for the purpose of measuring heights, and during the 
journey every opportunity should be taken of comparing them with 
mercurial barometers. 
In the majority of cases, aneroids, even when they have been in the first 
instance correctly graduated, do not read accurately against the mercurial 
barometer at diminished pressures, and will be found almost always to 
possess more or less considerable plus or minus errors. These errors are 
tolerably constant in good instruments, though they are frequently 
considerably augmented when low pressures have been experienced for a 
length of time. 
Aneroids should be treated with almost as much care as chronometers, 
and should not be allowed to dangle about the person, or to be shaken up 
in pockets. If the watch size is employed, they can be conveniently 
carried in extra watch pockets.* 
Measurement of Heights with the Aneroid: —To measure the difference 
in height between two stations, two instruments should be used, and 
* On this subject the traveller will do well to read Mr. E. Whymper’s book, 
4 How to use the Aneroid Barometer ’ (J. Murray, London), and his remarks 
on the “ Watkin Mountain Aneroid ” in The Geographical Journal , January, 
1899. 
