THE NATIONAL PHYSICAL LABOBATORY. 141 



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 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 meter. I could not give 

 the number of figures required. The meter is defined at the freezing 

 point of water, the yard at a temperature of 62° Fahr. When a yard 

 and meter scale are compared they are usually at about the same tem- 

 perature; the difficulty of the comparison is enormously increased if 

 there be a temperature difi^erence of 30° Fahr. between the tAvo scales. 

 Hence we require to know the temperature coefficients of the two stand- 

 ards. 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 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 meas- 

 ure from the fact that the English glass thermometer as ordinarily 

 made and used is incapable of the accuracy now demanded for scientific 

 investigation. 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, sa}', to .01 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. 



values of corkectioxs to the exglish glass thermom- 

 eter scale to give temperatures ox the gas 

 ther:\iometer scale fouxd by 



various observers. 



Temp. Rowland. Guillaunie. \Mebe. 



0° 0° 



10 —.03 —.009 -}-.0.3 



20 —.05 —.009 -h-OO 



