1901.] on the Aims of the National Physical Laboratory. 665 



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 attention, 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 lor 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 cases of tubes. On this point again further ex- 

 periments are needed. 



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

 other branches of science, difficulties connected with the measurement 

 of temperature are of the first importance. 



I was asked some little time since to state, 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 temperature of 62° F. When a 

 yard and a metre scale are compared they are usually at about the 

 same temperature ; the difficulty of comparison is enormously in- 

 creased 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 deter- 

 mination of coefficients of expansion as one of the investigations 

 which it is desirable that the Laboratory should undertake. 



Or, again, take thermometry. The standard scale of temperature 

 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 demanded 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 immediato 

 past history of the thermometer employed, and it is almost hopeless 

 to construct a table, accurate, say, to *01, which will give the differ- 

 ence 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. 



This is illustrated by Table III., which gives the differences as 

 found (1) by Rowland ; (2) Guillaume ; (3) ? Wiebe, between a Kew 

 thermometer and the air thermometer. 



