l20 THIRD REPORT— 18S3. 



bration of a needle, it has been customary to apply a correction 

 for differences in the temperatures at which the observations 

 may have been made. 



The horizontal intensity varying during the day, it becomes 

 a question whether this arises from a change alone in the direc- 

 tion of the force, or whether this change of direction is not 

 accompanied by a change in the intensity of the whole force. 

 In a communication to the Philosophical Society of Cambridge *, 

 I suggested that deviations, fi'om whatever cause, in the direc- 

 tion of the horizontal needle, were referrible to the deviations 

 which, under the same circumstances, would take place in the 

 direction of the dipping needle. Adopting these views. Captain 

 Foster infers, from observations made by him at Port Bowen, 

 on the corresponding times of vibration of a dipping needle, 

 supported on its axis and suspended horizontally, that the diur- 

 nal change in the horizontal intensity is due principally, if not 

 wholly, to a small change in the amount of the dip. The observa- 

 tions, however, do not indicate that the force in the direction of 

 the dip is constant. Captain Foster's obsei'vations at Spitzber- 

 genf show, more decidedly, the diurnal variation of this force : 

 there, its maximum intensity appears to have occun*ed at about 

 3^ 30"" A.M., and the minimum at 2^ 47™ p.m. ; its greatest change 

 amountino; to -J^ of its mean value. The maximum horizontal 

 intensity appears to have occurred a little after noon, and the 

 minimum nearly an hour after midnight ; but there is consider-r 

 able irregularity in the changes which it undergoes. It would, 

 however, appear, from these observations, that the variations hi 

 the absolute intensity were in opposition to those in the hori- 

 zontal resolved part of it ; so that the principal cause of the 

 latter variations must have been a change in the dip itself. 

 Captain Foster considers " that the times of the day when these 

 changes are the greatest and least, point clearly to the sun as 

 the primary agent in the production of them ; and that this 

 agency is such as to produce a constant inflection of the pole 

 towards the sun during the twenty-four hours." This is in per- 

 fect accordance with the conclusions I had previously drawn 

 from the experiments on A'-hich I founded the theory of the di- 

 urnal variation of the needle :j:, as I had shown that if the diur- 

 nal variation of the needle arise from the cause which I have 

 assigned for it, the dip ought to be a maximum, in northern la- 

 titudes, nearly when the sun is on the south magnetic meridian, 

 and a minimum when it has passed it about 130°. 



* Transactions of the Philosophical Society of Cambridffe, 1820. 



t Phitosophical Transactions, 1828. ' J Ibid. 1827, pp. 345, 349. 



