July i8, 1901 



NA rURh 



293 



the result surprising, when the question is looked at as a hydro- 

 dynamical problem ; the lines of fluid near a small obstacle 

 will differ from those near a large one, and the distribution 

 of pressure over the large area will not be uniform. Sir \V. 

 Siemens is said to have found places of negative pressure near 

 such an obstacle. As Sir J. Wolfe Barry has pointed out, if the 

 average of 56 lbs. to the square foot is excessive, then the cost 

 and difficulty of erection of large engineering works is being 

 unnecessarily increased. Here is a problem well worthy of atten- 

 tion, and about which but little is known. The same, too, may 

 be said about the second of the Board of Trade rules. What is 

 the effective surface over which the pressure is exerted on a 

 bridge ? On this again our information is but scanty. Sir B. 

 Baker's experiments for the Forth Bridge led him to adopt as 

 his rule, Double the plane surface exposed to the wind and deduct 

 50 per cent, in the case of tubes. On this point again further 

 experiments are needed. 



To turn from engineering to physics. In metrology, as in 

 many other branches of science, difficulties connected with 

 ihe measurement of temperature are of the first importance. 



I was asked some little time since to slate, to a very high order 

 of exactness, the relation between the yard and the metre. 

 I could not give the number of figures required. The metre 

 is defined at the freezing point of water, the yard at a temper- 

 ature of 62° F. When a yard and a metre scale are com- 

 pared they are usually at about the same temperature ; 

 the difficulty of comparison is enormously increased if 

 there be a temperature difference of 30° F. between the two 

 scales. Hence we require to know the temperature coefficients 

 of the two standards. But that of the standard yard is not 

 known ; it is doubtful, I believe, if the composition of the alloy 

 of which it is made is known, and in consequence Mr. Chaney 

 has mentioned the determination of coefficients of expansion as 

 one of the investigations which it is desirable that the Labor- 

 atory should undertake. 



Or, again, take thermometry. The standard scale of tempera- 

 ture is that of the hydrogen thermometer ; the scale in practical 

 use in England is the mercury in flint glass scale of the Kew 

 standard thermometers. It is obvious that it is of importance to 

 science that the difference between the scales should be known, 

 and various attempts have been made to compare them. But 

 the results of no two series of observations which have been 

 made agree satisfactorily. The variations arise probably in great 

 measure from the fact that the English glass thermometer, as 

 ordinarily made and used, is incapable of the accuracy now de- 

 manded for scientific investigations. The temporary depression 

 of the freezing point already alluded to in discussing the Jena 

 glass is too large ; it may amount to three- to four-tenths of a 

 degree when the thermometer is raised 100". Thus the results of 

 any given comparison depend too much on the immediate past 

 history of the thermometer employed, and it is almost hopeless 

 to construct a table, accurate, say, to 'oi, which will give the 

 difference between the Kew standard and the hydrogen scale, 

 and so enable the results of former work in which English 

 thermometers were used to be expressed in standard degrees. 



Table III.— Values of Corrections lo the English Glass Ther- 

 mometer Scale to give Temperatures on the Gas Thermometer 

 Scale found by various Observers. 



This is illustrated by Table III., which gives the dififerences 

 as found (i) by IRowland ; (2) Guillaume ; (3)?Wiebe between 

 a Kew thermometer and the air thermometer. 



It is clearly important to establish in England a mercury 



NO. 1655, VOL. 64] 



scale of temperatures which shall be comparable with the hydro- 

 gen scale, and it is desirable to determine as nearly as may be 

 the relation between this and the existing Kew scale. 



I am glad to say that in the first endeavour we have secured 

 the valuable cooperation of Mr. Powell, of the Whitefriars 

 Works, and that the first specimens of glass he has submitted to 

 us bid fair to compare well with 16'". 



Another branch of thermometry at which there is much to do 

 is the measurement of high temperature. Prof. Callendar 

 has explained here the principles of the resistance thermo- 

 meter, due first to Sir W. Siemens. Sir W. C. Roberts- 

 Austen has shown how the thermopile of Le Chatellier may 

 be used for the measurement of high temperatures. There is 

 a great work left for the man who can introduce these or similar 

 instruments to the manufactory and the forge, or who can im- 

 prove them in such a manner as to render their uses more simple 

 and more sure. Besides, at temperatures much over 1000' C. , the 

 glaze on the porcelain tube of the pryometer gives way. 



So far we have discussed new work, but there is much to be 

 done in extending a class of work which has gone on quietly 

 and without much show for many years at the Kew Obser- 

 vatory. Thermometers and barometers, wind gauges and other 

 meteorological apparatus, watches and chronometers, and many 

 other instruments are tested there in great numbers, and the 

 value of the work is undoubted. The competition among the 

 best makers for the first place, the best watch of the year, is 

 most striking and affords ample testimony to the importance of 

 the work. 



Work of this class we propose to extend. Thus, there is no 

 place where pressure gauges or steam indicators can be tested. 

 It is intended to take up this work, and for this purpose a 

 mercury pressure column is being erected. 



Again, there are the ordinary gauges in use in nearly 

 every engineering shop. These, in the first instance,' have 

 probably come from Whitworth's, or nowadays, I fear, from 

 Messrs. Pratt and Whitney or Browne and Sharpe, of America. 

 They were probably very accurate when new, but they wear, 

 and it is only in comparatively few large shops that means exist 

 for measuring the error and for determining whether the gauge 

 ought to be rejected or not. Hence arise difficulties of all 

 kinds. Standardisation of work is impossible. 



In another direction a wide field is offered in the calibration 

 and standardisation of glass measuring vessels of all kinds, 

 flasks, bu/ettes, pipettes, &c. , used by chemists and others. 

 At the request of the Board of Agriculture we have already 

 arranged for the standardisation of the glass vessels used in the 

 Babcock method of measuring the butter fat in milk, and in a 

 few months many of these have passed through our hands. 

 We are now being asked to arrange for testing the apparatus for 

 the Gerber and Leffman-Beam methods, and this we have 

 promised to do when we are settled at Bushy. Telescopes, 

 opera-glasses, sextants, and other optical appliances, are already 

 tested at Kew, but this work can, and will, be extended. 

 Photographic lenses are now examined by eye ; a photographic 

 test will be added, and I trust the whole may be made more 

 useful to photographers. 



I look to the cooperation of the Optical Society to advise 

 how we may be of service to them in testing spectacles, micro- 

 scope lenses and the like. The magnetic testing of specimens 

 of iron and steel, again, offers a fertile field for inquiry. 

 If more subjects are needed it is sufficient to turn over the 

 pages of the evidence given before Lord Rayleigh's Commission, 

 or to look to the reports which have been prepared by various 

 bodies of experts for the executive coinmittee. 



In electrical matters there are questions relating to the funda- 

 mental units on which, in Mr. Trotter's opinion, we may help 

 the officials of the Board of Trade. Standards of capacity are 

 wanted ; those belonging to the British Association will be 

 deposited at the Laboratory. Standards of electromagnetic 

 induction are desirable ; questions continually arise with regard' 

 to new forms of cells other than the standard Clark cell, and in 

 a host of other ways work will be found. 



I have gone almost too much into detail. It has been my 

 wish to state in general terms the aims of the Laboratory to 

 make the advance of physical science more readily available for 

 the needs of the nation, and then to illustrate the way in which 

 it is intended to attain those aims. I trust I may have shown 

 that the National Physical Laboratory is an institution which' 

 may deservedly claim the cordial support of all who are inter- 

 ested in real progress. 



