30 
CORRECTION OF THE COMPASSES, 
used in the construction, and especially in the guns 
of a ship of war ; now as the vessels of the Niger 
expedition were wholly of iron, the amount of attraction 
was so great as to render the compasses useless without 
the application of some corrective. This was done on the 
plan of Professor Airey, the Astronomer Royal, by placing 
two powerful bar-magnets in such positions, mth respect 
to the compass, as to counteract the effect of the mass 
of ii’on, and to coerce it into the correct magnetic 
direction. To ascertain the proper positions in which 
these magnets should be placed, the vessel was swung to 
all the points, in still water, comparing at the same time 
a compass on board with one on shore, and altering the 
position of the magnets till the compasses agreed on all 
the bearings. 
Each of the larger vessels had two engines, of thirty- 
five horse power ; that is, an aggregate power of sev^enty 
horses, and bunkers to contain coals for fifteen days of 
twelve hours^ steaming. The small vessel, one engine, 
of thii*ty-five horse power, and coals for ten days of 
twelve hours. The fire-places were adapted for burning 
wood. The engines being precisely similar in all these 
vessels, much space in stowage and expense were 
saved, as a smaller number of duplicate parts was 
required. 
It was not deemed advisable to have engines of great 
power ; because, in the first place, speed was not necessary 
in navigating an almost unknown river, where the 
numerous sand-banks and rocks required the utmost cir- 
