128 Transactions. 
To obviate this the reflecting surface was covered with an extremely delicate 
film of pure metallic silver by what is termed Browning’s process, produced 
by the decomposition of nitrate of silver by sugar of milk. After a few trials 
on another glass surface this process was successfully applied to the speculum, 
when the silver film having been polished to a fine surface by a wash-leather 
rubber and fine rouge, the speculum was ready for mounting. The mounting 
requires to be very accurate in order to give equal support so as to avoid the 
slightest flexure of the glass, but this was effected in a very simple manner by 
laying it on six freely-balanced points so arranged as to give equal support. 
Art. XITI.—On the Influence of Change of Latitude on Ships’ Compasses. 
By Captain EDWIN. 
[Read before the Wellington Philosophical Society, 6th November, 1872.] 
In all calculations concerning the magnetic character of ships, several co- 
efficients are used ; of these, five, namely, from A to E, are used to ascertain 
the approximate value, and the corresponding letters of the German alphabet 
are used to obtain the exact values : these co-efficients enable us to compute the 
alterations that take place on change of magnetic latitude, and enable us to 
compute and correct excessive deviations, and also the heeling error or the 
new magnetic character which becomes developed as the ship leansover. This 
error is due to vertical induction in soft iron, and though well known to exist 
is, except in vessels of war, taken but little notice of. It is most important, 
as the change in deviation due to its influence is very great, it having been 
found that, even in most carefully placed compasses, a vessel may have when 
upright an easterly deviation, but an inclination of a few degrees may change 
it to westerly ; this shows at once that, unless this peculiarity is allowed for, it 
will seriously affect the position of the ship. 
Suppose an iron ship is coming to Wellington from Lyttelton, and that soon — 
after leaving port a fair wind comes off the land, and being of good strength 
it leans the ship over, it is evident that iron which was before horizontal now 
becomes inclined and thus becomes magnetized by induction, the upper ends 
becoming north poles ; these poles now attract the south end of the compass 
needle, and consequently it approaches the higher side of the vessel, and the 
north point drops towards the lower side. The helmsman, who we will suppose 
has been told to steer north, finding that the vessel’s head is not in the given 
direction, brings the north point ahead again, and the result is that instead of 
making the desired course the vessel is steered to that side of it toward which 
the north point has dropped, and the captain finds that the vessel is not in the 
position he intended, but not being aware that this is the effect of heeling error 
