160 THE ANTARCTIC MANUAL. 
identical, then the force of gravity at the place is normal. If the 
tabular boiling point is higher than the observed boiling point, then 
the local force of gravity is less than the normal; if it is lower than 
the observed boiling point, then the local force of gravity is greater 
than the normal. In arriving at these conclusions, we postulate the 
complete exactness of the tables giving the relation between baro- 
metric pressure and the temperature of saturated steam, and of those 
giving the normal gravitational correction for latitude and for height 
above the sea. 
The use of the hypsometer along with the barometer may give 
valuable results at sea. It may give us a means of divining changes 
of depths of the ocean, especially in tropical latitudes where, when the 
sea is calm, all the conditions for such experiments are most favourable. 
On land, the subject has been taken up with great zeal by Professor 
Mohn,* the distinguished head of the Norwegian Meteorological Ser- 
vice. He has already published a preliminary report, and further 
reports from him will be expected with interest. In his hands this 
method will be thoroughly tested in all directions. 
It is not suggested that such observations should be made by the 
Antarctic Expedition. The margin of 0°-147 C. between the normal 
boiling point at the poles and at the equator shows that very fine 
thermometers would be required, and many other refinements are 
necessary. It is quite evident that the height of the barometer at 
0° C. can be arrived at with the requisite certainty only if the 
barometer has been hanging for at least a day in a room of uniform 
temperature. Of course the whole of the working part of the thermo- 
meter must be immersed in the steam. Correction for an exposed 
portion of the mercurial thread of the thermometer is quite in- 
admissible. The combined use of the barometer and the thermo- 
meter has been discussed at this length because it affords an 
opportunity of arriving at precise ideas of the fundamental principles 
of both instruments, and of the uses to which each may be legiti- 
mately put. 
A legitimate, and at the same time, an eminently practical use of 
the hypsometer is to replace the mercurial barometer in the deter- 
mination of the atmospheric pressure, and in the comparison of 
barometers at distant stations. On all camping excursions it is 
necessary to have the means of boiling water, and except under 
stress of circumstances, it is boiled night and morning. Saturated 
steam is therefore produced in the ordinary day’s work; it is only 
* Das Hypsometer als Luftdruekmesser, von I, Mohn, Christiania, 1899. 
