JANUARY 25, 1901.] 
to an elaborate program. One determina- 
tion will be made in October, 1901, and an- 
other in March, 1902, so that the first re- 
sults can be discussed before undertaking 
the second set of observations. Two pairs 
of observers, using four instruments, as 
nearly similar as possible, will be em- 
ployed. At each station the French and 
English observers will frequently exchange 
their instruments. The pairs of instru- 
ments and observers will be exchanged 
twice during each determination. The 
method employed will be submitted to the 
examination and approval of the Central 
Bureau of the Association. 
In this connection M. Albrecht called at- 
tention to the advantage of using the regis- 
tering micrometer of the Repsold system, 
which eliminates the personal equation of 
observers. 
Mr. Foerster spoke of errors resulting 
from the instability of the optical axes of 
instruments, caused by the movement of 
the objectives in their mounting. 
General Bassot emphasized the necessity 
of watching attentively the nature and con- 
stancy of the electric communication, whose 
variability may explain the marked differ- 
ences. 
M. Cornu exhibited and explained an ap- 
paratus to determine the zenith distance 
of a star culminating near the zenith. On 
motion of M. Hirsch the Association re- 
solved that a similar apparatus should be 
constructed and studied in one of the great 
observatories, and that M. Cornu, contin- 
uing his useful studies, should realize a 
portable zenitho-nadiral apparatus for use 
in temporary observatories, permitting nu- 
merous and easy determinations of latitude 
at all the points of a geodetic net. 
Upon the invitation of the Association, M. 
Guillaume, of the International Bureau of 
Weights and Measures, presented a very in- 
teresting communication concerning nickel- 
steel. He remarked in 1896 that nickel-steel, 
SCIENCE. 
151 
with 30 per cent. nickel, had an expan- 
sion sensibly less than platinum, and since 
then has conducted most interesting studies 
concerning these alloys. He dwelt upon 
their singular magnetic, thermometric and 
mechanical properties, and stated that a cer- 
tain alloy with 35 per cent. to 36 per cent. 
nickel possesses an expansion ten times less 
than that of platinum and twenty times less 
than that of brass. He then passed to the 
uses of the alloy for geodetic purposes. He 
considers it impracticable to use the alloy 
in the construction of standards of the first 
order, where perfect stability and invari- 
able dimensions within the limits of preci- 
sion measures are demanded for long periods 
of time, on account of certain changes which 
have been observed as a function of time. 
In geodesy the question presents a different 
aspect, and it suffices in practise that the 
length varies very little, say a millionth part 
in a year, and that the change should be 
very regular. The small coefficient of ex- 
pansion, the malleability, the feeble oxi- 
dation, and the homogeneous character of 
these alloys all increase their usefulness in 
the construction or use of geodetic instru- 
ments. 
The applications of nickel-steel have 
passed the experimental stage and entered 
the practical field in the two methods of 
measuring bases actually in use, viz., those 
of rigid bars and wires. 
The problem of constructing a base bar 
has been solved in a very satisfactory man- 
ner in the one actually being constructed 
for the Geographic Service of the French 
Army, from the plans of MM. Benoit and 
Guillaume. The bar, which is four meters 
long, has a section in the form of an H in- 
scribed in a square of 40 millimeters. It 
is enclosed in a case, made of an alloy of 
aluminum, with openings for necessary ob- 
servations. It rests in its case on two nor- 
mal points of minimum flexure. It weighs 
50 kilos, case included. 
