MAGNETISM. 



31 



with several whether equally or un- 

 equally whether circular or libratory ; 

 if circular, about what centre ; if libra- 

 tory, after what manner ; are secrets as 

 yet utterly unknown to mankind, and 

 are reserved for the industry of future 

 ages*.' 



6. Diurnal Changes of Variation 

 and Intensity. 



(121.) Independently of the changes 

 already noticed, the position of the mag- 

 netic needle is liable to certain slight 

 variations, according to the time of the 

 day, and also according to the season of 

 the year. The daily change in the va- 

 riation was discovered in 1724 by Mr. 

 George Graham, and has been confirmed 

 by many subsequent observers. This 

 change, however, is exceedingly minute, 

 and requires the most careful observa- 

 tion, and the most delicate instruments 

 to render it sensible, even in the hori- 

 zontal needle ; and it is still more diffi- 

 cult of detection in the dipping needle, 

 which does not admit of the same de- 

 gree of delicacy of suspension. 



(122.) Professor Barlow, to whom the 

 science of magnetism is so much in- 

 debted for its more recent improvements, 

 has devised a mode of rendering these 

 diurnal oscillations much more percep- 

 tible, by diminishing the ordinary direc- 

 tive power of the needle, through the 

 influence of one or two magnets, placed 

 in such positions with respect to the 

 needle as to counteract, and thereby 

 neutralize, as it were, the terrestrial ac- 

 tion. The effect of the ordinary action 

 being thus removed, he was led to ex- 

 pect that the extraordinary cause, what- 

 ever it might be, which produced the 

 daily variation, would exhibit its effects 

 much more perceptibly ; and thus not 

 only the amount of the changes it pro- 

 duces, but also the period of their taking 

 place, and of the maximum of their ope- 

 ration, might be ascertained with great 

 precision. These expectations have been 

 amply realized by the success of his own 

 experimental researches, and also by 

 those of Mr. Christie, which are de- 

 tailed in several papers in the ' Philoso- 

 phical Transactions t.' 



(123.) The general result of the ex- 

 periments of the latter of these obser- 

 vers was, that the deviation of the hori- 

 zontal needle from the mean position was 

 easterly during the forenoon, and was 



* Phil. Trans, for 1683, p. 220. 

 f For 1823, 1825, and 1827. 



of greatest amount at about eight o'clock, 

 thence returning quickly to its mean po- 

 sition, which it attained between nine 

 and ten o'clock, after which it became 

 westerly ; at first increasing rapidly, so 

 as to reach its maximum at about one 

 o'clock in the afternoon, and then slowly 

 receding during the rest of the day, and 

 arriving at its mean position by about 

 ten o'clock at night. The state of the 

 weather, and more particularly that of the 

 temperature, had considerable influence 

 on the nature and extent of the changes. 



(124.) Mr. Christie remarks that the 

 changes which are observed to take place 

 cannot be explained by a change in the 

 directions alone of the terrestrial forces, 

 but that their characters agree, as nearly 

 as we can possibly expect, with the effects 

 that would take place from an increase 

 of intensity at the time that the direction 

 deviates towards the west. 



(125.) The occurrence of diurnal 

 changes of intensity at Christiana in 

 Norway have been ascertained by 

 M. Hansteen ; the same conclusion being 

 deducible from his observations of the 

 vibrations of a needle very delicately 

 suspended ; and also from those of Mr. 

 Christie, made with a different appara- 

 tus, and by a totally different method. 



(126.) M. Hansteen found that the 

 minimum intensity occurs about half 

 past ten o'clock in the morning, that is, 

 about two hours after the westerly devi- 

 ation has commenced, and the maximum 

 intensity at half-past seven in the even- 

 ing, that is, about the same time after 

 the return towards the east. Mr. Chris- 

 tie* found that the terrestrial magnetic 

 intensity is the least between ten and 

 eleven o'clock in the morning ; the time, 

 nearly, he observes, when the sun is on 

 the magnetic meridian ; that it increases 

 from this time until nine and ten o'clock 

 in the evening ; after which it decreases, 

 and continues decreasing, during the 

 morning, until it attains its minimum 

 already stated. 



(127.) The general dependence of 

 these variations of magnetic position on 

 diurnal changes of temperature is suffici- 

 ently apparent from the results hitherto 

 obtained. But the prosecution of the in- 

 quiry involves considerations of another 

 kind, connected with a subject we have not 

 yet touched upon, namely, electro-mag- 

 netic and thermo-magnetic phenomena. 



(128.) The mean diurnal changes of 

 variation were found by Mr. Canton to 



* Philosophical Transactions for 1825, p, 51, 



