4 PRESIDENT’S ADDRESS. 
before the Association from this Chair; they have been exhaustively 
demonstrated by Sir William Huggins from the Chair of the Royal 
Society, and now a special guild! exists for their enforcement upon the 
mind of the nation. 
These considerations appear to warrant me in following the healthy 
custom: of so many previous Presidents —viz., of confining their remarks 
mainly to those departments of science with which the labours of their 
lives have been chiefly associated. 
The Science of Measurement. 
Lord Kelvin in 1871 made a statement from the Presidential Chair 
of the Association at Edinburgh as follows: ‘Accurate and minute 
measurement seems to the non-scientific imagination a less lofty and 
dignified work than the looking for something new. But nearly all the 
grandest discoveries of science have been the reward of accurate measure- 
ment and patient, long-continued labour in the minute sifting of numerical 
results.’ 
Besides the instances quoted by Lord Kelvin in support of that 
statement, we have perhaps as remarkable and typical an exemplification 
as any in Lord Rayleigh’s long-continued work on the density of nitrogen 
which led him to the discovery of argon. We shall see presently that, 
true as Lord Kelvin’s words are in regard to most fields of science, they 
are specially applicable as a guide in astronomy. 
One of Clerk Maxwell’s lectures in the Natural Philosophy Class at 
Marischall College, Aberdeen, when I was a student under him there, in 
the year 1859, ran somewhat as follows :— 
A standard, as it is at present understood in England, is not a real standard at 
all; it is a rod of metal with lines ruled upon it to mark the yard, and it is kept 
somewhere in the House of Commons. If the House of Commons catches fire there 
may be an end of your standard. A copy of a standard can never be a real standard, 
because all the work of human hands is liable to error. Besides, will your so-called 
standard remain of a constant length? It certainly will change by temperature, it 
probably will change by age (that is, by the rearrangement or settling down of its 
component molecules), and I am not sure if it does not change according to the 
azimuth in which it is used. At all events, you must see that it is a very impractical 
standard—impractical because, if, for example, any one of you went to Mars or 
Jupiter, and the people there asked you what was your standard of measure, you 
could not tell them, you could not reproduce it, and you would feel very foolish. 
Whereas, if you told any capable physicist in Mars or Jupiter that you used some 
natural invariable standard, such as the wave-length of the D line of sodium vapour, 
he would be able to reproduce your yard or your inch, provided that you could tell 
him how many of such wave-lengths there were in your yard or your inch, and your 
standard would be available anywhere in the universe where sodium is found, 
That was the whimsical way in which Clerk Maxwell used to impress 
1 The British Science Guild. 
