METEOROLOGY AND CLIMATOLOGY. 
21 
index . The phenomenon known as surface-tension gives to the free 
surface of any liquid the properties of a tough film, and the smaller 
the area of a free surface is, the greater is this effect of surface-ten¬ 
sion. Hence it is that the inner surface of the column of alcohol is 
not penetrated by the glass index, but draws the index with it back¬ 
wards towards the bulb. As soon as the temperature begins to rise, 
the alcohol once more flows past the index towards the farther end 
of the tube. The end of the index farthest from the bulb remains 
opposite the mark on the stem indicating the lowest temperature which 
had occurred since it was last set, and this reading must be taken with¬ 
out touching the thermometer. 
To set the index it is only necessary to tilt the bulb end of the tube 
upwards, when the index will slide down by its own weight until it 
comes in contact with the inner surface of the end of the column of 
alcohol. 
Care of a Minimum Thermometer .—The chief dangers to which a 
minimum thermometer are liable are three—(1) the index being shaken 
into the bulb, (2) the index being shaken partly or wholly out of the 
column of spirit, and sticking in the tube, and (3) the column of spirit 
becoming separated or a portion of the spirit evaporating into the upper 
end of the tube. 
The thermometer should be so constructed as to make it impossible 
for the index to get into the bulb, and this should be seen to before 
purchasing. When any of the other derangements occurs the natural 
instinct of an observer is to immerse the thermometer in warm water 
until the spirit entirely fills the tube, and then allow it to cool. The 
only drawback to this simple method is the almost inevitable bursting of 
the bulb and destruction of the thermometer. This method should never 
be attempted; but if the warning were not given, the idea would be sure 
to occur to the observer some time or other, and he would proceed to 
destroy his thermometer with all the fervour of a discoverer. The only 
satisfactory way to rectify a deranged minimum thermometer is as 
follows: 
If the column is separated, but the index remains in the spirit, grasp 
the instrument firmly by the upper end and swing it downwards with a 
jerk (as in the case of the mercurial thermometer mentioned on p. 14). 
If the index has been shaken out of the spirit and remains sticking in the 
