S46 
Mr Barlaw'^s Discoveries in Magnetism. 
It is obvious, that although these laws have been pointed out 
with reference to the circles supposed to be drawn on the iron ball, 
or circumscribed sphere, they actually appertain to the needle it- 
self ; for when the compass is situated in any particular latitude 
and longitude with respect to the circles of the ball, the centre of 
the latter is similarly situated with reference to a sphere conceived 
to circumscribe the compass, having its centre coinciding with the 
centre of the needle ; and hence the rules become immediately ap- 
plicable to the determination of the local effect of a ship’s guns, 
viz. find by experiment or otherwise, the centre of attraction of 
all the ship’s iron ; then in any part of the world where the 
dip is known, and at any direction of the ship’s head, find the 
latitude and longitude of the centre of attraction with reference 
to a sphere circumscribing the compass, as supposed above; 
and the efiect of the attraction may then be computed by means 
of the above rules, the deviation in any one instance having 
been previously determined. The only thing that can be con- 
sidered as doubtful in the above rule is this : Does the circle of 
no attraction every ivliere incline to the horizon at an angle 
equal to the complement of the dip ? Mr Barlow has little 
doubt that it does, but, to be assured of the fact, he conceives 
that experiments must be repeated in different parts of the 
world. 
Having ascertained the law of deviation as it regarded posi- 
tion, and which he found to hold in the most irregular masses 
of iron, Mr Barlow next proceeded to ascertain the law as it re- 
gards distance, and he found by the most unexceptionable re- 
sults, that, all things else being the same, the tangents of the 
angles of deviation are reciprocally proportional to the cubes of 
the distances. 
And again, that, caeteris paribus^ the tangents of deviation are 
directly proportional to the cubes of the diameters of the iron 
halls or shells^ by which the deviation is produced. 
By combining these laws with those given above, Mr Bar- 
low has found, that the whole maybe expressed by the formula, 
Sin . cos Z ^vhere A is the angle of deviation, a 
tan A == 
A.d 
the latitude, I the longitude, D the diameter of the ball or 
shell, d the distance of the centre of attraction from the pivot 
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