702 PROCEEDINGS OF SECTION E, 
No. 5.—THE DETERMINATION OF HEIGHTS BY 
BAROMETRIC METHODS. 
By Toomas WaLker Fow ter, M.C.E., F.R.G.S., F.G.S. . 
(Read, Wednesday, January 12, 1898.) 
THE geographer, surveyor, or civil engineer frequently requires 
to ascertain the relative heights of various points upon the earth’s 
surface or their absolute heights above sea-level. The most accu- 
rate method of making such determinations is that termed by 
geodesists “ precise levelling,” being a method somewhat similar 
to that adopted by civil engineers for determining the relative 
heights of points close together, and known as “spirit levelling.” 
Work of this character is, however, both tedious and expensive, 
and is but rarely resorted to, more especially when the points to 
be connected are a considerable distance apart, or separated by 
obstacles, such as dense forests or precipitous mountains. In 
such cases even the civil engineer’s “ spirit levelling ” becomes 
very costly, and, for preliminary investigations, has to be 
abandoned. 
Two other systems remain, one by ascertaining the atmospheric 
pressure at the different places whose altitudes are required, and 
from such pressures deducing the heights, the other by measuring 
the angles of elevation or depression which the objects subtend 
from each other or from other points at which such objects are 
visible. The first method is that generally termed ‘‘ barometric 
levelling” and the second “ trigonometric levelling.” The writer 
has for several years, when opportunity offered, made experiments 
with both methods, and he hopes that the results of his baro- 
metric experience may be useful to others. 
Under the head of “barometric levelling” it will be well to 
consider—Ist, the instruments used ; 2nd, the methods of taking 
the observations ; and 3rd, the methods of computing the results. 
The instruments used are necessarily some description of baro- 
meter, supplemented by thermometers, for the purpose of ascer- 
taining the air temperature. The barometer may be one of 
several types, mercurial, aneroid, or boiling-point thermometer. 
For mountain work the mercurial barometer is generally of either 
the siphon or the Fortin type, most of the writer’s work (so far 
as mercurials are concerned) having been done with the former. 
The great advantage claimed for the siphon barometer is that 
as the diameters of the tube at the upper and lower limbs are the 
same the capillary depressions in the two limbs would be equal, 
