Be NaS eg PEE eee ae 
L. Waldo— Papers on Thermometry. 451 
Remarks by Professor Rowland on the preceding letter, in a com- 
munication dated Johns Hopkins University, April 29,1 1881. 
Through the kindness of Dr. Waldo, I have been pride to see 
met 
Bical thermometry, but only on that part which should be use- 
ul to me in measuring differences of temperature within the limits of 
0 and 45° C. And sol merely made a study of thermometers, 
n the course of my investigation I discovered the fact that the 
Geissler thermometers, especial y the one I Sue efit departed 
more from the air thermometer ee any woe Now the Geiss- 
And this a abe was so ie amoun ting to ov r 0°3 C., for 
allax, and recording his results to thousandth of a degree, and all 
this on a thermometer having an error of 0 °3.C.! As Dr. Thiesen 
remarks: If one is to compare his thermometer with the air ther- 
mometer, the amount of correction is of little importance: but de- 
parture from the air thermometer is certainly not a recommendation 
and, indeed, must introduce slight errors. The most accurate 
Bai, 8 which one can make on an air thermometer will vary 
several hundredths of a degree. 
ence we can never use with accuracy the direct comparison 
With the air thermometer but must ite ab the difference of the 
two instruments by some formula of the 
panini ae 
Should we take an infinite number of terms this formula would 
8 
¢~-) 
€ number rms curve of differences becomes smooth 
and smoother and the formula expresses less and less the irregu- 
larities of the experiment. e number of terms to be used is a 
matter of judgment, and this point I sought to determine by the 
use of the observations of en and others. The rejection of 
the higher powers of ¢ is more or less of an assumption founded 
on the fact that we are reasonably certain that the curve of differ- 
ences between the mercurial and the air thermometer is a smooth 
curve. It is evident that the less oe correction to be introduced 
the sig the rejection of the higher powers of ¢ will affect our 
results, 
