DETERMINATION OF HEIGHTS BY BAROMETRIC. METHODS. 705 
in steam the reading of the thermometer can be ascertained with 
the greatest ease to 0:01° Fahr., but if placed in the ordinary 
water-bath for comparison with other thermometers the writer 
has been unable to get a steady reading no matter how vigorously 
the stirring operations are carried on. Further, when compared 
in the water-bath at (say) 200° Fahr., the thermometer being 
vertical, its bulb is subject to a considerably greater pressure than 
it would be if immersed in steam at the same temperature, and 
consequently the thermometer reading may be expected to be 
somewhat higher. The apparatus with which the writer is experi- 
menting consists of a boiling-point apparatus with air and steam 
tight joints, and glass tube as a thermometer chamber, the whele 
being connected with a water-jet exhaust, and the barometer of 
the aneroid testing chamber. The supply of steam can be regu- 
lated by control of the gas-jet heating the apparatus, while the 
exhaust can also be readily adjusted so that there is no difficulty 
in getting absolutely steady thermometer and barometer readings, 
and hence determining the pressure of the steam corresponding 
to a given reading of the thermometer. It is of course unneces- 
sary for the present purpose to know what the absolute tempera- 
ture of the steam is, and indeed it would seem desirable to 
graduate thermometers used for determining heights by the boiling 
point of water, not in degrees Fahr., or cent., but in inches or 
millimetres of mercury. 
The apparatus in which the thermometer is placed when in use 
consists of a boiler to which a jacketed-tube (of length sufficient 
to contain the whole of the thermometer) is attached. A light 
wire carrier is placed in the tube, and prevents the bulb of the 
thermometer touching the sides of the jacketed-tube. A vent- 
tube placed near the top of the tube discharges the steam at one 
side, the thermometer passing through an india-rubber washer at 
the top. In use the thermometer is inserted as far as possible in 
the jacketed-tube, so that stem and bulb shall be at the same 
temperature, and about half an inch of the mercury column alone 
need be visible. The bulb of the thermometer should be about 
4 inches (as a minimum) above the water in the boiler so as to be 
fairly free from the splash of the water when boiling. The spirit- 
lamps burning vaporised spirit seem the most suitable for use with 
the apparatus in the field, though if care be used the ordinary 
camp fire may be used. The heat and flame must be applied to 
the boiler alone, however, and not allowed to reach the sides of 
the steam jacket, or a broken thermometer will probably be the 
result. 
The use of maximum thermometers has been suggested for 
taking boiling-point observations, but the writer considers them 
unsuitable for the purpose in view of the alteration in the reading 
due to variations in the temperature of the stem (vide note by 
De Ne 
