174 OBSERVATIONS OF THE TEERESTRIAL 



in which t and t' are the two temperatures, and T and T' the 

 corresponding times of vibration. We find 



Cyl. L (a). T = 242 8 -50, T - T' = 1 8 '45, t - t' = 23-5 



a = -000254 

 Cyl. L (J). T' = 293 s -44, T - T' = 1"78, t-f= 24-5 



a = -000248. 



It is to be observed that these cylinders were made at the same 

 time, and were therefore probably tempered to the same degree ; 

 and to this circumstance we may, with much probability, ascribe 

 the close agreement in the values of the constant which determines 

 the effects of temperature upon the force of the needle. 



No observations were made to determine directly the effects of 

 temperature upon the other needles employed in the course of 

 these observations; and, in correcting the results obtained with 

 them, the coefficient employed by M. Hansteen, viz., -00017, has 

 been that adopted. The standard temperature (t), to which all 

 the results contained in the following pages are reduced, is 60 Fahr. 



4. All that we know of the diurnal variations of the intensity of 

 the horizontal force is due to M. Hansteen and Professor Christie. 

 These writers agree in fixing the hours of minimum intensity at 

 10^- A. M. The intensity then increases, and attains its maximum, 

 according to Professor Christie, at about 7| p. M. The amount of 

 this maximum is 1*0024 in summer, the minimum intensity being 

 unity; but this amount, as well as the hour of its occurrence, 

 changes with the season. Of the law according to which the force 

 varies between its two limiting values, we know nothing ; and it 

 is therefore impossible, in the present state of our knowledge, to 

 apply a correction for these variations. It was proposed to evade 

 this difficulty, in the ensuing observations, by observing at a fixed 

 hour. To this limitation, however, it was found impracticable to 

 adhere, and the results still remain uncertain by the amount of the 

 diurnal change. 



5. The variations of the magnetic force give rise to another 

 and still graver class of errors. The least experience in obser- 

 vations of this nature is enough to prove that the horizontal in- 

 tensity is, from some cause or other, subject to irregular fluctuations ; 

 and these fluctuations, like those of the barometer in our climates, 

 are much more considerable than the regular horary changes. It 

 seems probable that these variations in the intensity of the hori- 



