PRESIDENT’S ADDRESS. 5 
great principles upon us. We all laughed before we understood ; then 
some of us understood and remembered. 
Now the scientific world has practically adopted Maxwell’s form of 
natural standard. It is true that it names that standard the metre ; but 
that standard is not one ten-millionth of the Earth’s quadrant in length, 
as it was intended to be ; it is merely a certain piece of metal approxi- 
mately of that length. 
It is true that the length of that piece of metal has been reproduced 
with more precision, and is known with higher accuracy in terms of many 
secondary standards, than is the length of any other standard in the 
world ; but it is, after all, liable to destruction and to possible secular 
change of length. For these reasons it cannot be scientifically described 
otherwise than as a piece of metal whose length at 0° C. at the epoch 
A.D. 1906 is =1,553,164 times the wave-length of the red line of the 
spectrum of cadmium when the latter is observed in dry air at the 
temperature of 15° C. of the normal hydrogen-scale at a pressure of 
760 mm. of mercury at 0° C, 
This determination, recently made by methods based on the interfer- 
ence of light-waves and carried out by MM. Benoit, Perot, and Fabry at 
the International Bureau of Weights and Measures, constitutes a real 
advance in scientific metrology. The result appears to be reliablewithin 
one ten-millionth part of the metre. 
The length of the metre, in terms of the wave-length of the red line in 
the spectrum of cadmium, had been determined in 1892 by Michelson’s 
method, with a mean result in almost exact accordance with that just 
quoted for the comparisons of 1906 ; but this agreement (within one part 
in ten millions) is due in some degree to chance, as the uncertainty of the 
earlier determination was probably five times greater than the difference 
between the two independent results of 1892 and 1906. 
We owe to M. Guillaume, of the same International Bureau, the dis- 
covery of the remarkable properties of the alloys of nickel and steel, and from 
the point of view of exact measurement the specially valuable discovery of 
the properties of that alloy which we now call ‘invar.’ He has developed 
methods for treatment of wires made from this alloy which render more 
permanent the arrangement of their constituent molecules. Thus these 
wires, with their attached scales, may, for considerable periods of time 
and under circumstances of careful treatment, be regarded as nearly 
invariable standards. With proper precautions, we have found at the 
Cape of Good Hope that these wires can be used for the measurement of 
base lines of the highest geodetic precision with all the accuracy attain- 
able by the older aed most costly forms of apparatus ; whilst with the 
new apparatus a base of 20 kilometres can be measured in less time and 
for less cost than one of a single kilometre with the older forms of 
measurement. 
