230 Mr Griffiths, On Thermonietric "Fixed Points." [Nov. 28, 



The numbers in Col. v. are obtained directly from the platinum 

 thermometer readings whose constants are as follows : 



^ = 1793-434, 8 = 15105, 



^ = 1293^947 hence ^ atl00 °- 4-9195. 

 FT= 499-487' hence dt atlUU ~ 4yiy5 ' 



R -1793434* 



.-. b.p. = 100 + 



4-9195 



and as the numbers given in Col. III. are the differences from 

 1792-546, by substitution we get 



1 -1 r\r\ Oj '888 



b -P- = 100+ «iT95' 



where d is the difference as given in Col. III. 



The last figure of the numbers in Col. III. was read directly 

 from the box-vernier without any estimation of intervals — the 

 galvanometer showing a distinct deflection for a difference of '001. 



Since -j- at 100° = 4 - 92 approximately, 



it follows that a difference of 5 in the last figure corresponds to 

 0°"001, hence the scale even as regards pressure is nearly twice as 

 open as that of a barometer whose vernier reads to "01 mm. and 

 thus, provided there is no lag, a more accurate determination of 

 the pressure can be made by this thermometer than by observing 

 the barometer itself. 



The barometer observations were taken by one observer at the 

 same instant as the other read the resistance indicated on the 

 box. 



An inspection of this table will I think convince the most 

 sceptical that the temperature lag of the steam may be dis- 

 regarded. It appears astonishing that the temperature adjustment 

 should be so rapid, especially when a sudden fall such as that 

 shown from 2.30 to 3 p.m. supra takes place. Here the cooling 

 can only be caused by excess of evaporation, and when one reflects 

 on the mass of copper, &c, bounding the enclosure, the rapidity of 

 adjustment is surprising. 



I should mention that the hypsometer is of a somewhat 

 unusual form and that special screens are used both below and 

 above the thermometer bulb. 



* These values of the constants were obtained subsequently, and are quite 

 independent of the observations on Oct. 7th. 



