KErORT ON THE MAGNETISM OF THE EARTH. 1 19 



creasing, at Christiana, until ten or eleven o'clock in the morn- 

 ing, when it attains its minimum, and then increases until four 

 or five o'clock after noon, when it appeared to reach its maxi- 

 mum*. By observing, at different times of the day, the direc- 

 tion of a horizontal needle thrown nearly at right angles to the 

 meridian, by the action of two powerful magnets, placed in the 

 meridian, passing through its centre, after correcting the ob- 

 servations for the effect of changes of temperature on the in- 

 tensity of the force of the magnets, I found that at Woolwich 

 the terrestrial horizontal intensity decreased until 10'' 30™ a.m., 

 when it reached its minimum, and increasing from that time, 

 attained its maximum about 1^ 30™ p.M.f. This agreement, in 

 results obtained, by totally independent methods, removes all 

 doubt respecting the diurnal variation of the horizontal force. 

 The difference in the time of the maximum in the two cases 

 may be accounted for, independently of the difference in the 

 variation at the two places of observation, by the circumstance 

 that no correction for the effect of temperature on the time of 

 vibration is made in Professor Hansteen's observation. As no 

 such correction had hitherto been made, it must have been con- 

 sidered that differences in the temperature at which observations 

 were made had little influence on the intensity of the vibrating 

 needle ; but in the communication containing these observations, 

 I pointed out the necessity of such a correction |; and since 

 then, in deducing the terrestrial intensity from the times of vi- 



* Edinburgh Philosophical Journal, vol. iv. p. 297. 



+ Philosophical Transactions, 1825, pp. 50 & 57. An inconvenience attending 

 the method which I employed is, that the observations require a correction for 

 temperature which is not very readily applied, as will Tie seen by referring to my 

 paper ; but this might in a great measure be obviated, by rendering the tempera- 

 ture of the magnets employed always the same previous to observation. If, how- 

 ever, in order to retain the needle in its position nearly at right angles to the me- 

 ridian, torsion were applied instead of the repulsive forces of magnets, the correc- 

 tion for temperature would be neai-ly reduced to that due to the effects produced 

 on the intensity of the needle itself by changes of temperature. But even this 

 method is not without objection ; for the sensibility of the needle depending 

 upon the number of circles of torsion requisite to bring it into the proper posi- 

 tion, if a wire were employed, unless very long, its elasticity would be impaired 

 by more than two or three turns ; and it is doubtful whether a filament of glass 

 of moderate length would bear more than this without fracture. I had pro- 

 posed to the late Captain Foster, previous to his last voyage, that he should de- 

 termine the horizontal intensity at different stations, and also its diurnal changes 

 by this method, and had a balance of torsion constructed for him for the purpose ; 

 but as the instrument is extremely troublesome in its adjustments, I consider 

 that the many other observations which he had to make did not allow him time 

 for the extensive use of this instrument which he had proposed. It is, however, 

 very desirable that it should be ascertained how far this method is applicable. 



X Philosophical Transactions, 1825. 



