August i, 1907 J 



NA TURE 



319 



Inaugural Address by Sir David Gill, K.C.B., LL.D., 



D.Sc, F.R.S., Hon. F.R.S.E., 6ic., President of the 



Association. 



To-NlGiiT, for the first time in its history, the British 

 Association meets in the ancient city of Leicester ; and it 

 now becomes my privilege to convey to you, Mr. Mayor, 

 and to the citizens generally, an expression of our thanks 

 for your kind invitation and for the hospitable reception 

 which vou have accorded to us. 



Here in Leicester and last year in York the Association 

 has followed its usual custom of holding its annual meet- 

 ing somewhere in the United Kingdom ; but in 1905 the 

 meeting was, as you know, held in South Africa. Now, 

 having myself only recently come from the Cape, I wish 

 to take this opportunity of saying that this southern vi'sit 

 of the Association has, in my opinion, been productive of 

 much good : wider interest in science has been created 

 amongst colonists, juster estimates of the country and its 

 problems have been formed on the part of the visitors, 

 and personal friendships and interchange of ideas between 

 thinking men in South Africa and at home have arisen 

 which cannot fail to have a beneficial influence on the 

 social, political, and scientific relations between these 

 colonies and the mother country. We may confidently 

 look for like results from the proposed visit of the 

 Association to Canada in igog. 



One is tempted to take advantage of the wide publicity 

 given to words from this chair to speak at large in the 

 cause of science, to insist upon the necessity for its wider 

 inclusion in the education of our youth and the devotion 

 of a larger measure of the public funds in aid of scientific 

 research ; to point to the supreme value of science as a 

 means for the culture of those faculties which in man pro- 

 mote that knowledge which is power ; and to show how 

 dependent is the progress of a nation upon its scientific 

 attainment. 



But in recent years these truths have been prominently 

 brought 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 ' e.xists for their enforcement upon the mind of the 

 .'lation. 



These considerations appear to warrant me in follow- 

 ing the healthy custom of so many previous presidents — 

 viz., of confining their rcmarivs mainly to those depart- 

 ments of science with which the labours of their lives 

 have been chiefly associated. 



The Science of Measurement. 



Lord Kelvin in 187 1 made a statement from the presi- 

 dential 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 measurement 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 

 field's of science, they are specially applicable as a guide 

 in astronomy. 



One of Clerk Maxwell's lectures in the Natural Philo- 

 sophy 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 English, 

 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 some- 

 where 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 tempera- 

 ture, it probably will change by age (that is, by the re- 

 1 The British Sciince Guild. 



arrangement or settling down of its component molecules), 

 and I am not sure if it docs 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 repro- 

 duce 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 great principles upon us. We all laughed 

 before we understood ; then some of us understood and 

 remembered. 



Now the scientific world has practically adopted Max- 

 well's form of natural standard. It is true that it names 

 that standard the metre ; but that standard is not one- 

 millionth of the earth's quadrant in length, as it was 

 intended to be ; it is merely a certain piece of metal 

 approximately 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. igo6 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 interference of light-waves and carried out by MM. 

 Perot and Fabry at the International Bureau of Weights 

 and Measures, constitutes a real advance in scientific 

 metrology. The result appears to be reliable within 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 1006 ; 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 

 ten times greater than the difference between the two 

 independent results of 1892 and 1906. 



We owe to M. GuiUaume, of the same International 

 Bureau, the discovery 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 allov 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 

 attainable by the older and 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. 



The Great African Arc of Meridian. 



In connection with the progress of geodesy, time only 

 permits me to say a few words about the Great African 

 arc on the 30th meridian, which it is a dream of my life 

 to see completed. 



The gap in the arc between the Limpopo and the 

 previously executed triangulation in Rhodesia, which I 

 reported to the Association at the Johannesburg meeting 

 in iqo5, has now been filled up. My own efforts, at 



NO. 1970, VOL. 76] 



