368 Mr. J. R. Napier on Ships' Compasses. 



heavy. Hence, the method already described has suggested itself as 

 being simpler and more economical, and by experiment more powerful 

 than any I had tried. I jiave preferred it. I therefore recommend this 

 particular arrangement of needles for its simplicity, its economy, and 

 its durability ; for this same experimental card, after lying about for 

 two years in every direction without any care being taken to have its 

 poles protected, yet retains about twenty per cent, more power than one 

 of Sir Snow Harris's recently magnetized. I presume from this example, 

 therefore, that neither Mr. Cameron, who at the last meeting objected to 

 compound needles on account of their tendency to lose their power, nor 

 Captain Walker, who objects for the same reason, have literally followed 

 Dr. Scoresby's instructions, or they would not have failed to make 

 powerful and enduring compound needles. 



So far from the compound needles of Dr. Scoresby losing their power, 

 I have reason to know, from information received from that gentleman, 

 that they are decidedly the most enduring of all others, of which he 

 favours me with the following evidence : — 



A compound needle of his construction, comprising six plates of thin 

 steel, six inches in length, was put up in July, 1836, which was then 

 found to have about double the power of any compass needle of the same 

 weight which, at that time, he could meet with. Another of four plates, 

 the same length, weighing altogether 579 grains, was put up August, 1839, 

 which was considerably stronger than the other. These needles were 

 placed in a case near to each other, mutually protecting, but were many 

 times taken out for examination or experiment. After about sixteen 

 years they were tested as to their residual powers, when the former was 

 found to have lost only 6"6 per cent, and the other less than 6 ! Another 

 needle of four plates was attached to a card, in February, 1839, and placed 

 in a box quite ■unprotected, and exposed to various magnetic influences from 

 magnets lying about. In May, 1841, the power (which had not been well 

 recorded) was examined, and it was then singularly powerful. It was 

 re-examined in the beginning of the present year, 1855, and was found, 

 though entirely unprotected, to have lost only 7|- per cent, in 13| years ! 



The Liverpool makers, Gray and Keen, seem to consider weight a 

 necessary element of a steady card, if not of a powerful one, for their cards 

 have a great weight of brass, paper, and talc, and very little magnetic 

 steel in their construction. It will be allowed, however, that if weight 

 alone, without much directive power of magnetic steel, is all that is 

 necessary to make a steady card, a much less complex arrangement of 

 parts could easily be contrived. Gray's card is about the same weight as 

 the Admiralty standard card, and has only about half its power. In 

 other words, Gray's card might be pointing steadily one or two points 

 from the truth on account of its want of power to overcome the friction, 



