32 



MAGNETISM. 



differ at different seasons of the year, 

 being greatest in June and least in De- 

 cember *, and he has given the results of 

 his observations in a tabular form. Mr. 

 Gilpin investigated this subject at a later 

 period, and gives also tables of the diur- 



nal changes of variations in each month, 

 which he found to be very different in 

 different years. The following table 

 contains the results of these different ob- 

 servations *. 



Mean Diurnal Changes of Variation. 



Months. 



Canton In 

 1769. 



January' 7' 8" 



February 8 58 



March 11 17 



April 12 26 



May 13 



June 13 21 



July 13 14 



August 12 19 



September 11 43 



October 10 36 



November 8 9 



December . . .658 



CHAPTER IV. 

 Theories of Magnetism. 



1. Mechanical Theories. 



(129.) IN the general view we have 

 now given of the present state of our 

 knowledge with regard to magnetism, we 

 have strictly confined ourselves to the 

 statement of facts, unmixed with hypo- 

 thetical speculations as to the nature of 

 the powers from which they proceed. 

 We have solely endeavoured to genera- 

 lize the facts, as far as their nature and 

 extent would warrant. The result has 

 been their reduction to a small number, 

 such as the mutual attractions and re- 

 pulsions of magnetic iron according to 

 certain laws, the induction of these 

 properties on other iron, the differences 

 in the capacities of receiving and of re- 

 taining these properties, existing in dif- 

 ferent kinds of ferruginous bodies, and 

 the magnetic influence of the globe of 

 the earth. 



(ISO.) But the human mind is so 

 constituted as to refuse being restrained 

 within the boundaries of a rigid induc- 

 tive philosophy. Incited by an irresisti- 

 ble desire of exploring the secrets of 

 Nature, it scruples not as to the means 

 of forcing her to disclose them; and 

 borne on the wings of imagination and 

 conjecture, presses forwards with an 

 eagerness which often betrays it into 

 courses widely deviating from the truth. 

 Yet good is often found to result from 



* Phil, Trans, for 1759, p. 39S. 



these erratic excursions of our faculties : 

 they infuse fresh interest into the pursuit 

 of knowledge ; they inspire with the hope 

 of success ; they invigorate those powers 

 which must be exerted to attain it. The 

 spark which kindles a train of light is 

 sometimes struck out in the conflict of 

 discordant speculation; and amidst a 

 multitude of attempts, some effort, more 

 happy than the rest, elicits an important 

 discovery. No great or comprehensive 

 fact in science was ever established, 

 without being preceded by a bold though 

 sagacious conjecture. Hypothesis of 

 some kind or other is invariably the pre- 

 cursor of truth. 



(131.) Magnetism, ever since it occu- 

 pied the attention of philosophers, has 

 been a fertile soil for hypothesis. That 

 a shapeless and unorganized lump of 

 metal should have the power of drawing 

 towards itself another equally rude and 

 unfashioned piece lying at a distance, 

 or of forcing it to move away, as if both 

 were alive, and animated by some prin- 

 ciple of active sympathy ; and that all 

 this should take place, whatever may be 

 the number or kind of the intervening 

 bodies, nay even in the apparent absence 

 of any connecting medium, are pheno- 

 mena of too remarkable a nature not to 

 excite in us a lively curiosity to learn 

 their cause. No wonder that the an- 

 cients, who had but imperfect notions of 

 the real objects of philosophy, were im- 

 pressed with a vague notion of their 

 connexion with immaterial agency. Mag- 



* Phil. Trans, for 1806, pp. 416, 417. 



