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GEOPHYSICS: W. BOWIE 
Proc. N. a. S. 
In North America great progress has been made in coordinating the 
geodetic work of the several countries. Some years ago, there was adopted 
for the triangulation net of the United States what was called the United 
States Standard Datum. This datum had its designation changed to 
North American Datum when, in 1913, it was adopted by Canada and 
Mexico. It is hoped that within a few years a continuous triangulation 
will be extended into Alaska from the United States and then that terri- 
tory can also have its triangulation placed on the same datum as the rest 
of the continent. 
A datum may be defined as the adopted latitude and longitude of some 
station of the triangulation and the azimuth of a line of the triangulation 
radiating from that station. It also includes the dimensions of the refer- 
ence spheroid, on which the triangulation is computed. 
The advantages of a single datum are, first, that it makes it possible 
to compute the figure of the earth with greater accuracy and, second, that 
it avoids the confusion existing in the maps at the frontier of any country 
when the adjoining countries base their surveys and maps on different 
datums. Whenever the datums are different, boundary monuments have 
different geographic positions on the maps of the two countries and there 
are overlaps, gaps, and offsets in the maps, which frequently result in 
controversies over the boundary line. 
One of the greatest needs in Europe to-day, from a geodetic standpoint, 
is the adoption of one datum for the whole of its area. This would in- 
volve a tremendous amount of revision of the computation and adjust- 
ment of the triangulation nets of the several countries, but the resulting 
benefits would far offset the cost of the work. 
Trigonometric leveling is a branch of geodetic work which, though im- 
portant, is not as much so as triangulation and precise leveling. It is 
employed in determining the elevations of the triangulation stations in 
order that the lines observed may be corrected to what they would be 
if the observations were made at sea level. Theoretically, trigonometric 
leveling should furnish data from which to compute the shape of the 
geoid, but this is impracticable owing to our inability to know 
the exact value of the atmospheric refraction at the instant of observa- 
tion. Trigonometric leveling is of value in determining the elevations 
of mountain peaks used as stations or those outside of the main scheme 
of the triangulation. It is by this method that the elevations of Pikes 
Peak, Mt. Everest, Mt. McKinley and many others, have been deter- 
mined. 
A branch of geodetic work which has been receiving great attention 
during the past decade or more is the determination of the intensity of 
gravity. For a number of years observations were made using re- 
versible pendulums in order to determine the absolute value of gravity. 
Following this, observations were made with what were called invariable 
