AND ITS TEMPERATURE CORRECTION. 



71 



The following short series, taken at random from a great nmnber of observa- 

 tions, at once prove the truth of this conclusion. From a compailson of a few of 

 the observations, it was found that an increase of 1° Fahr. was equivalent to an 



s 



increase of 0.076 in the time of vibration. 



The last column for each series gives the times corrected by this quantity to 

 50° Fahr. That the correction obtained is only approximate, will, together with 

 errors of observation, account for much of the discrepancies in the corrected 

 quantities. 



Table III. 



Observations for the Time of Vibration of the Balance Needle in the Vertical 

 Plane at different Temperatures. 



It should be remarked, that the series for January 1844 is not comparable 

 with the following series, as an adjustment of the instrument occurred in that 

 month ; neither, indeed, are the other series comparable with each other, from the 

 circumstances given in Table I. 



7. To take one of the most marked cases from this table, it wUl be seen that 

 the observed times of vibration on January 23d and 31st 1845, differ nearly two 

 seconds, while the corrected times do not differ one-tenth of a second. 



8. While an inequality in the expansion of some parts of the needle would 

 alter its sensibility by elevating the centre of gravity, it seems very doubtful 

 if there is any thing in the form of the needle which is at all likely to render 

 this supposition suflBcient. An alteration in the position of the centre of motion 

 would produce a like effect ; and as the position of the needle depends, to some 

 extent, on its temperature, it is necessary to shew whether position or tempera- 

 ture only is the cause of the differences in the times of vibration. Had the read- 

 ings for the position of the needle been given with Table III., it would have been 

 evident from these alone that the differences were not due to differences of posi. 



