358 
-MB. EYA2s’S OX THE DE^HATIOX OP THE COr^IPASS 
ApPE>’'DIX. 
No. I. 
The Admiralty Standard Compass. 
The chief points in the construction of this instrument are the following : — 
1 . The howl is constructed of stout copper with the ^iew to calm the vibrations of the 
needle, and the intersecting point of the axis of its gimbals is made to coincide with the 
point of suspension of the card, and also with the centre of the azimuth cii'cle. 
2. The azimuth circle is accurately graduated to minutes of arc, and may be used in 
addition on shore for surveying purposes: thus when accurate magnetic bearings are 
required, the zero of the circle may be adjusted to the magnetic north, shown by the 
card, and then clamped ; any number of magnetic bearings may then be obtained round 
the circle: or by adjusting the zero of the circle to any given object and clamping the 
compass to its stand, the angles of objects round the horizon may be observed and read 
off to the nearest minute. 
In observing Amplitudes and Azimuths, the bearings are read from the card with- 
out reference to the azimuth circle, the card being graduated to 20 minutes. 
3. The magnetic needles employed are compound bars, formed of lamiuEe of that kind 
of steel (clock-spring) which has been ascertained by numerous experiments to be 
capable of receiving the greatest magnetic power. Each compass has two cai'ds, A and J ; 
the former is used at all times, except in stormy weather with much motion in the ship, 
when the heavy or J card is substituted. 
Each card is fitted with four needles fixed vertically and equidistant on a light frame- 
work of brass screwed to the card; the pair of central needles are 7‘3 inches long, and 
the pair of external ones 5’ 3 inches, the whole weight of the A card being 1525 grains*. 
The pivots for the needles are pointed with “ native alloy,” as being harder than steel, 
and not subject to corrosion by exposure to the atmosphere; the ruby cap of the card is 
worked to a form to suit these points. 
4. The impressions of the cards are taken after the paper has been cemented to the 
mica plate forming the basis ; distortions from slirinking are thus prevented and a more 
perfect centering attained. 
Finally. The cards and needles are adjusted to tlie magnetic meridian at the Com- 
pass Observatory for Her Majesty’s Navy at Woolwich, a place free from the local 
influence of iron. The various adjustments for centering, and the elimination of errors 
* In order that a vibration communicated to a cai’d about a diameter intermediate between the X. and S., 
and the E. and W. diameters, may not give rise to a “ wabbling” motion, the moment of inertia of the card 
about these two diameters should be tlie same. It was shown by Mr. Archibald Smith that this is the 
case with any two parallel needles of uniform thickness forming chords of the same circle, if their extre- 
mities are separated by G0° ; consequently, ivith any four such needles, if their extremities are sep.arated 
by 30° ; and the needles of the Admiralty Standard compasses have been imiformly so ai’r.auged. 
