366 Ml-. J. R. Napikb on Ships' Compasses. 



published in his " Magnetical Investigations," there could be no doubt 

 that a compass with a number of thin needles would be much more 

 powerful than if the same weight of steel formed one solid bar needle. 

 I was surprised, however, at finding it stated in Sir Snow Harris's " Rudi- 

 mentary Treatise of Magnetism" (page 147), that no sufficient reason can 

 be assigned for the employment of from three to five compound magnetic 

 bars of costly and difficult construction, supposing it were proved from 

 the evidence of experience as well as theoretically, that a single and 

 simple edge-bai" needle is even more than adequate to any required 

 practical purpose. The object in using more than one needle (as noticed 

 in page 141) is evidently a greater directive force, &c. This advantage, 

 however, as Professor Barlow considers, " cannot be obtained mthout an 

 increase of weight of steel, and, as a necessary consequence, a greater 

 amount of friction on the pivot of suspension. Unless therefore the directive 

 force increase in a greater ratio than the loss by friction and wear of the 

 centre, little advantage is gained." 



To satisfy myself I had a compass made, which I conceived would 

 combine the good qualities of most of the compasses I had read about. 

 I adopted Scoresby's compound needles made of clock springs, placed 

 them for economy flat on the card in two sets, foiu- in each set, as the 

 objection to the use of one needle on the flat is I conceive much diminished 

 where there are four, and still more so where there are eight needles. 

 I placed these two sets of needles at the distances calculated by Mr. 

 Archibald Smith, viz. 60°, shown by him to be the position (of the 

 needles) in which the compass card might pitch or roll in all positions 

 without affecting its direction. I tested the power of the card, with its 

 needles complete on it, by the torsion balance, as I did not know how to 

 apply Scoresby's deviation method where there were two sets of parallel 

 needles. This method, when the torsion of wire is divided by the weight 

 of card, gives I conceive a true measure of the powers which different 

 cards would have of indicating correctly by overcoming friction, &c., 

 when placed on their proper pivots. 



The card with its needles was nearly of the same weight as one of Sir 



Snow Harris's, made by Lilley of London. The result of trials made in 



January, 1853, was as follows : — 



Sir Snow Harris's single) ^^^ , ^ _ ,n,^o, cn^o^ • r- • 



1, 1 1 V 840 grs. was deflected 90 by 29f torsion of wire, 

 bar needle and card, ) ° j ^ 



And the Scoresbyj gSS grs. was deflected 90° by 48^° torsion of wire. 



needle and card of) 

 So that as the weights of the cards are about equal, their relative powers 

 are about 29f to 48J, or 100 to 162. This compound needle card has 

 therefore about 62 per cent, more power than Sir Snow Harris's single bar 

 needle card. A recent trial with a different torsion wire of the same ex- 



