• On Terrestrial Magnetism. 5 



of our knowledge of the changes that occur in tlie variation, tliat 

 the situation of the Hnes before mentioned on the chart cannot 

 be predicted for any distant future period: and consequently the 

 method ut present fails. By comljining the dip with the voria- 

 iion, a much greater degree of certainty will be given to it when 

 the Jaws of both are better understood: and perhaps it will be 

 still further improved bv adding the infcnsili/ ; or the num- 

 ber of vibrations which each of these needles makes in a given 

 time: a propertv that was first noticed bv our in'jje'.uous coun- 

 tryman Mr. George Graham, in the Philosophical Transactions 

 for IJ'Iiy, p. ."3.32. At present, however, little or notliingcau 

 be done to forward the purpose in question, for want of sufiicient 

 observations and data, on whicii to establish a permanent theory. 



It will probably be asked, \Miy should we take pains to bring 

 this method to perfection, which at best can oidy be an approxi- 

 mation, when we possess so many others that are founded on 

 principles not liable to uncertainty ? The ansv/er is plain and 

 cogent : It ought on no account to be neglected, whilst there 

 exists the least probability of its ever being applied to anv use- 

 ful purpose, or whilst a single ()ropertv of so curious a subject is 

 not thoroughly understood. By attempting to prove the exist- 

 ence of an annual parallax of the earth, the aberration of light 

 and the Jiutation of the earth's axis were discovered : and is it 

 possible to foretell that nothing will be found oat by endea- 

 vouring to render magnetism of ns.e to seamen ? Perhaps it 

 may be found to form some connecting link in the chain of 

 unexplained processes that are alwavs going forward in the for- 

 mation and decomposition of mineralogic substances in the 

 interior of the earth ; or perhaps it may account for certain 

 actions in the arrangement of their particles, especially when 

 ferruginous matter enters into the combination. The cultiva- 

 tion of any department of knowledge affords plea'^ure : but to 

 examine the operations of nature, to reduce them to system, and 

 IG hring her laws to light, is certaiidv tiie most exalted exercise 

 which the human mind, in its present imperfect state, is capa!)le 

 of enjoying. It the longitude cannot be found bv it, there v. ill 

 1)6 a gratitication in having the cause of its failure sati-factoriiy 

 explained. Till then it cannot be r<!linquished without regret. 

 Perhaps even the attempt to show its impracticability may elicit 

 some new idea, or lead us to combine it with some extraneous 

 principle or science, which may render it effectual for the pur- 



Iti the Atlantic Occsm within n trifle by the dcaH rockoniii-;, to |)rocec(l 

 aloii^ the shortest and most expeditious track \>y informini; tlic ni vdiat va- 

 riation tlity are to allow in the courses whicli they ought to steer. 



A 3 pose. 



