232 Mr Griffiths, On Thermometry, "Fixed Points." [Nov. 23, 



" It is time to come to some determination as to the value 



of the normal pressure — on which indeed all thermal units of 

 measurement ultimately depend. At present we define standard 

 pressure as that of 76 cm. of mercury at 0° in Paris — a complicated 

 and roundabout method which depends on a local value of gravity, 

 a temperature, and a density 



" The natural standard of pressure — 10 6 dynes per sq. cm. — is, as 

 is well known, very nearly the same as the present standard of 

 76 cm. I find that a column of mercury 75 cm. (three-quarters of 

 a metre) high at 0° at sea-level in latitude 45° exerts a pressure 

 of (1 — '00007) megadyne/cm. 2 , for most purposes identical with a 

 megadyne (in lat. 45° 1' identical). 



" I would propose then that the megadyne per sq. cm. be formally 

 adopted for the standard pressure to which to refer properties of 

 gases. The fact that it is f the pressure of a metre of mercury at 

 0° in lat. 45° is convenient and may be rendered accurate either 

 by altering infinitesimal^ the height, the temperature, or the 

 latitude. Or, the term " atmosphere " might be kept for it. 



" In this method instead of speaking of a vacuum of so many 

 millionths of an atmosphere we should say so many dynes per sq. 

 cm. The advantages are that the unit is an absolute one, that it 

 is nearly the same as that now in general use and that its value is 

 probably much nearer the time-mean of barometric height in the 

 majority of laboratories than 76 cm. 



" Supposing that the absolute unit of pressure be adopted, the 

 100° on the new thermometer would be the temperature of steam 

 under one megadyne per sq. cm., which — using data given in 

 Everett's "Units" — works out to 99"62° C. The new degrees 

 would therefore be "9962 times the present ones. 



" Taking again the number given in the B.A. Report for diminu- 

 tion of specific heat of water per 1° as "00036, and supposing it to 

 hold backward from 10° to 0° (which, however, can scarcely be the 

 case) the unit of heat (4 - 2 joules), would be measured by the heat 

 necessary to raise 1 gram of water from 0° to 1°. This last result, 

 if really true, is a curious coincidence and would be an argument 

 in favour of the change here proposed. It would define the heat 

 unit as at present, and at a temperature at which accuracy is in 

 general most easily attained. 



" It may be objected that the alteration of the thermometric 

 scale puts the proposal outside the limits of what is practical. To 

 this I would observe that there are two classes of people to be 

 considered, the general public and the scientific world. To the 

 first it would make no difference, the thermometers used by them 

 would scarcely show the difference and as a rule they do actually 

 err by amounts much larger than the change proposed. In 



