84 



TEMPERATURE CORRECTIONS. 



18. Table X contains data relating to the temperature correction to the Vertical Force. They were 

 obtained by intercom paring the temperatures recorded by a mercury thermometer adjacent to the Vertical- 

 Force instrument, at the times when the sheets were put on and taken off', with the readings of the 

 temperature trace and Vertical-Force curve at the beginning and end of each sheet. Sometimes, of course, 

 the natural values of V at the beginning and end of a day's trace differ considerably owing to magnetic 

 disturbance. Again, at times when temperature was changing rapidly it is improbable that the temperature 

 of the mercury thermometer, the metallic thermometer and the Vertical-Force magnet were really identical. 

 Uncertainties also arose from discontinuities in the temperature trace and Vertical-Force curve. 



Comparing the changes during the 2-t hours in the temperature trace with the corresponding changes 

 in the readings of the mercury thermometer, one obtained the number of millimetres in the temperature 

 trace answering to 1 C. The mean results thus obtained from the curves of individual months form the 

 first column of Table X. Taking everything into account, I concluded that from March, 1902, to the end 

 of February, 1903, there was no evidence of any real change in the scale of the metallic thermometer. 

 Accordingly the mean value 1 6 mm. per 1 C. was accepted for these twelve months. 



Subsequent to February, 1903, for some unknown reason, the temperature scale was much less open and 

 at the same time more variable. This continued until nearly the end of the year, when the scale again 

 reverted to nearly its original value. During the last three months a common value of 14'8 mm. per 1C. 

 was accepted, but from March to September, 1903, use was made of the individual monthly values actually 

 found. 



TABLE X. Vertical Force. Temperature Correction. 



Comparing the daily changes in the temperature trace with the corresponding changes in the Vertical- 

 Force ordinate, one got for each month assuming the effects of magnetic disturbances to neutralise one 

 another the apparent change of Vertical-Force ordinate in millimetres answering to a change of 1 mm. in 

 temperature trace ; while comparing the daily changes in the readings of the mercury thermometer with 

 the corresponding changes in the Vertical-Force ordinate, one got the apparent change of Vertical-Force 



