530 Correspondence on Magnetism of Ships, [Nov. 30, 



sensibility and steadiness it is to be understood that the needle is freely to 

 submit to the earth's magnetic force, with power sufficient to steadily obey 

 that force under the varying motions of a ship, without the aid of friction 

 or mechanical impediment, steadiness or rather sluggishness produced by 

 the latter causes being obtained at the expense of accuracy. 



" Simplicity of construction, so that repairs can be effected by an ordi- 

 nary skilled mechanic, must be deemed a qualification of merit. 

 " 2. The chief points to be attended to in construction are, 

 " (a) Great directive power of the needle, with little weight, and conse- 

 quently little friction on the point of suspension. 

 " (b) Permanency of the magnetic power of the needle, 

 "(c) Accurate adjustment of the several parts of the compass. 



This comprises (1) the magnetic axis of the needle coinciding 

 with the north and south points of the card. (2) The inter- 

 secting point of the axes of the jimbals of the bowl coinciding 

 with the point of suspension of the card. (3) The accurate cen- 

 tering of the point of suspension within the bowl. (4) The per- 

 fect impression of the card, so that the centering and marginal 

 divisions are not distorted by shrinking or other causes. 

 " 3. The advantages of a compound system of needles compared with a 

 single needle. 



" These arc, (1) greater directive power being obtained with the same 

 weight. (2) The needles can be placed on their edge, whereby there can 

 be no alteration of their magnetic axes, a condition frequently found in 

 flat bar needles. (3) By placing one (or two) pairs of equal parallel 

 needles with their ends 60 (or 30) apart, the ' wabbling ' motion com- 

 mon to single bar needles is avoided ; and the following remarkable pro- 

 perty also exists with this arrangement of the needles : 



" "When magnets or soft iron are placed as correctors of the larger devia- 

 tions due to the iron of the ship, unless the needle (where a single bar is 

 employed) be very short compared to the distance of the disturbing magnet 

 or iron, a deviation is introduced depending on the length of the needle. 

 This deviation disappears with the compound arrangement. 



" Proceeding from general principles to details, the following are the 

 chief points to be attended to in the construction of a Standard Compass. 



"1. The bowl to be constructed of pure copper, of substantial thick- 

 ness, and the part adjacent to the needle increased in solidity, by an extra 

 copper ring, the ends of the needle being permitted to work as close to 

 the ring as consistent with freedom of motion. 



"2. The needles to be fitted on the compound system (one pair to be 

 deemed sufficient), and efficiently tempered and magnetized. 



"3. The sight-vane to be arranged so as to turn freely in azimuth 

 without moving the compass-bowl or causing disturbance to the card. It 

 should be attached to a graduated circle, so as to show the angle between 

 the ship's head and any celestial object as measured on the horizon without 



