128 Tranflaction8. 



To obviate this tlie 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 tine 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 efiected in a very simple manner by 

 laying it on six freely-balanced points so arranged as to give equal support. 



Art. XIII. — On the Injluence of Clmnge of Latitude on Ships' Compasset. 



By Captain Edwin. 



[Head 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 leans over. 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 afiect 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 ofl" 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 



