22 
HINTS TO TRAVELLERS. 
upper part of the tube, or if a little spirit has volatilized into the top of 
the tube and cannot be shaken down by the first method, a quantity of 
spirit should be passed into the upper end of the tube by grasping the 
thermometer by the bulb end of the frame and swinging in the same way. 
When the index is immersed or the drop of volatilized spirits joined on 
to the column, the first process of swinging by grasping the upper end of 
the tube will bring the instrument into working order. After any opera¬ 
tion of this kind the thermometer should be kept in a vertical position 
bulb downwards, to allow the spirit adhering to the sides of the tube to 
drain back completely. Then the thermometer should be brought into 
the horizontal position and set by allowing the index to slide down to the 
end of the column of spirit. The end of the column of spirit farther from 
the bulb should always show the same temperature as the dry-bulb 
thermometer. If it should be observed to read a degree or two lower, it 
will be found that some of the spirit has volatilized and condensed at the 
end of the tube. 
The minimum thermometer should be exposed to the air four or six 
feet from the ground under a screen or roof, like that described on p. 15, 
so that it is not exposed to the open sky, and the ground under the 
shelter should be covered with grass or leaves, not on any account left 
bare. The loss of heat by radiation of the ground to the open sky will 
produce a night temperature much lower than that of the air a few 
feet above the ground, and a radiation thermometer with blackened 
bulb is often employed laid on the grass and exposed to the sky to 
measure this effect. Travellers, however, can rarely be expected to 
make observations of such a kind, as the instrument is one of extreme 
delicacy. 
Maximum Thermometers .—Maximum registering thermometers are filled 
with mercury, and are less liable to go out of order than spirit-ther¬ 
mometers. The simplest and best form for use by travellers is Negretti 
and Zambra’s. Its principle is very simple. When the temperature 
rises and the mercury in the bulb expands, it forces its way along 
the stem in the usual manner; but there is a little contraction in the 
tube just outside the bulb which breaks the column as the temperature 
begins to fall, and so prevents the mercury in the bulb from drawing 
back the thread of mercury from the tube. The thermometer is hung 
horizontally, and the end of the mercury farthest from the bulb always 
