APPENDIX. 
369 
in a gale of wind it is often extremely difficult to take the 
temperature exactly. 
Lastly, the copper or brass apparatus in common use is 
so clumsy that most travellers after a time take to the use 
of an old soup-tin^ and boil the thermometer in water 
instead of in steam. 
The apparatus which I have invented, and of which a 
woodcut is here given_, has the following advantages : — 
I. The water is made to boil by the heat of a candle, 
which gives a uniform source of heat, and a measured 
quantity of water (about \ oz.) being used, the results are 
extremely uniform. " Composition^^ candles are easily 
carried, and are more obtainable almost everywhere. The 
piece of candle, about two inches long, which is used for 
boiling the thermometer, is pushed up by means of a spiral 
spring, as in an ordinary carriage lamp. 
II. My thermometer is a self-registering maximum one — 
i.e., the mercury is broken in the tube, and kept so by the 
presence of a very minute portion of air above the mercury, 
or rather between the piece which forms the index and the 
mercury filling the bulb. I find that a self-registering instru- 
ment of this sort can be carried to some distance without 
moving the index, so that it may be boiled in the dark, and 
be carried to one^s tent some miles distant in order to be read. 
III. The brass tube in which the thermometer is boiled 
forms the case for carrying it in the pocket. 
To prevent the detached piece of mercury which forms 
the index from being shaken down into the bulb, Mr. Hicks 
makes the thermometer with a constricture just above the 
bulb ; but should the index in spite of this arrangement get 
into the bulb, the instrument may again be made self- 
registering in the following way. To facilitate this there 
is a small bulb at the top of the thermometer tube, and the 
mercury must be very cautiously heated over a candle until 
this top bulb is partially filled ; it is then to be inverted 
and shaken, when a piece of mercury gets detached and 
falls into the upper bulb, and when the instrument cools, 
this detached piece of mercury again forms the index. 
The advantages of a good boiling-point thermometer over 
B B 
