Dr. Mills's Researches on Thermometry* 5 



Zero Movements. 



With regard to the investigation of the movement of the 

 zero, we would remark that it would have been simpler to use 

 a formula which expressed the use of the zero directly. The 

 expression adopted by Prof. Mills is 



y=Au x + B/3 x , 



where as measures the time, and A, B, a and j3 are constants. 

 The quantity y is called the total remaining ascent, and "A + B 

 represents the total ascent/' i. e. the whole rise up to some 

 fixed time. 



The rise of the zero (Z) up to the time x is therefore evi- 

 dently 



Z = A + B-?/; 



and the tables would have been more useful had this quantity 

 Z been given instead of y. 



The account of the movement of the zero with temperature 

 is so involved that it constitutes a riddle which it took us 

 some time to read. We quote the opening sentence : — 



" It has long been known that the immersion of a thermo- 

 meter into boiling water almost invariably lowers the zero. 

 The only consecutive series of observations of the effect of 

 temperature that I have been able to find is due to Henrici *. 

 His results are readily comprised in the equation 



# = 2'100(-981)*--099(l-360)*; 



y being the total remaining ascent, and the unit of x being 

 10° C. The starting-point of the observations was 50° C; 

 and the depressions were consecutively observed at every 10° 

 to 100°. 



"Table vii. 



X. 



Zero observed. 



y- 



y calc. 







0-00 



200 





1 



-0-10 



1-90 



1-92 



3 



-0-25 



1-75 



1-73 



4 



-040 



1-60 



1-61 



5 



-060 



1-40 



1-45 



Probable error of a single 



comparison . 



.... -023 



The formula here employed is of the same form as that used 

 above, except that one of the terms is negative. The quantity 

 y is, as before, called " the total remaining ascent.' ' The 

 effect of heating the thermometer having been to depress its 



* "Pogg.-4»w. 1.251," 



