168 I^IESSES. SMITH AND EVANS ON THE EEEECT OF THE LENGTH OF 
cases of iron vessels which have been hitherto examined, positive the arrangement fur- 
nishes a correction obeying the requhed law. The results of an experiment originally 
made for the purpose of demonstrating the danger of this arrangement of two com- 
passes, will be found in the well-known work on the ‘ Deviations of the Compass,’ by 
the late Captain Edwaed Johnson, R.N., F.RS., Superintendent of the Compass De- 
partment of the Royal Navy, Table V., 2nd edition, p. 69. In this experiment the 
compasses were the Admiralty Standard, placed two feet apart, and the effect of the 
reciprocal action of the compasses on each other was to produce a negative quadrantal 
deviation in the one of 8° 6', in the other of 8° 18', without producing any other appre- 
ciable deviation. As the amount of deviation produced is inversely as the cube of theh 
mutual distance, these compasses, at the distance of 2^ feet, would have produced a 
negative quadrantal deviation on each other of about 4°, and would therefore have cor- 
rected the usual amount of quadrantal deviation found in iron ships without the intro- 
duction of soft iron correctors. 
A similar experiment, made by Mr. Evans with two single needles of 6 inches in 
length, placed 1 foot 6 inches apart, gave a negative quadrantal deviation of 6° 40', with 
an octantal deviation of +68'. Two such compasses so arranged would therefore correct 
a quachantal deviation of the largest class, without the introduction, even with single 
needles, of a material octantal error, and this octantal error may be corrected by the 
employment of two compasses, each with two or four needles arranged in the manner 
before described. 
It is perhaps not unworthy of rerhark, as an incident in the history of this subject, 
that an experiment which is cited by Captain Johnson as a warning against placing 
compasses near each other as in the ordinary double binnacle, and which was the cause 
of an Admiralty order that compasses should in no case be brought within 4^ feet of 
each other, should in the course of time have become the means of correcting an error 
which the change in the material and mode of construction of ships has brought into 
prominence. 
The defects in this method of correcting the quadrantal deviation appear to be — 
1. The two compasses being in different positions, may, particularly in ships built head 
east and west, have different independent deviations. Some care must therefore be used 
to select a place at which the magnetism of the ship is nearly constant within the area 
occupied by the two compasses ; and this mode of correction will probably be found 
more applicable to large ships, in which the magnetism is more uniform, than m small 
vessels, in which there may be great changes in the magnetism with small changes in 
position. 
2. The possible decrease in the force of the needles. In this respect the defect is 
common to this, with every mode of correcting deviations by magnets. In the case of 
the Admiralty Standard compass, the proved permanency of the magnetism is such as 
.to show that this defect may be disregarded. 
* See Mr. Evans’s paper, Philosophical Transactions, 1860, p. 337. 
