On Terrestrial Magnetism. 7 



pending marine barometers in gimbals, just above the centre of 

 gravity, may, under certain modifications, be applied with ad- 

 vantage to these compasses, in keeping them steady when the 

 sea is rough. The mean of a number of observations will give 

 the dip and variation with tolerable exactness, even when little 

 dependence can be placed on a single attempt. They can always 

 be taken on land, with all the accuracy that the nature of the 

 subject admits, provided the ship remain at anchor a sufficient 

 time for the purpose. 



Exclusive of any view to utility, the subject, like all the other 

 operations of nature, is in itself extremely curious ; and, it is pre- 

 sumed, will amply repav, with interesting and amusing novelties, 

 whatever trouble may attend its cultivation. With so great a pro- 

 bability of its becoming useful in the important art of conduct- 

 ing ships from one part of the world to another, or of its lead- 

 ing to useful discoveries in the (economy of nature, by explaining 

 the causes of important changes in the mineralogic and geo- 

 logic departments, it cannot but recommend itself strongly to 

 the attention of all those who feel pleasure in the advancement 

 of useful scientific knowledge. 



I remain, sir. 



Yours, &c. 

 Christ's Hospital, Jan. 1, 1817. T. S. EvANS. 



On the Laws of Terrestrial Magnetism in different Latitudes, 



W E have mentioned elsewhere, that the dip, the variation^ 

 and the intensity of the magnetic force, are not the same in all 

 parts of the earth. We now possess all the necessary processes 

 for giving the present state of these phaenomena with exactness. 

 It is only requisite to carry with us to different places the same 

 magnetic needle, or others that are capable of being compared 

 together, and to observe the three elements or data above named 

 in those places. 



This great experiment was undertaken about the year 1700, 

 by the celebrated astronomer Dr. Halley, to whom the English 

 Government gave the command of a vessel for the purpose 

 of carrying him and his instruments to different parts of the 

 globe. But the researches of Dr. Halley having for their ob- 

 ject the determination of the longitude of places by means of 

 the variation of the compass, he confined himself principally 

 to observe this element, which unfortunately appears to be the 

 most variable of the three. So that, if we wish to describe 

 the present state of terrestrial magnetism, wc are obliged to 



A 4 liuve 



