TERRESTRIAL MAGNETISM.—MURPHY. 305 
the compasses in the Royal Navy were mere lumber and ought 
to be destroyed. Since then many improved varieties of ships’ 
compasses have been introduced. The most remarkable and, as 
shewn by trial, most satisfactory, form of compass is said to be 
that patented in 1876 by Sir William Thomson. 
The conditions that chiefly affect the use of the mariner’s 
compass are those of the declination and variation to which 
I have already referred. The magnetism of the ship itself, or 
that induced in it by the earth’s magnetic force, was first ob- 
served in 1772-1774 by Mr. Wales, the astronomer of Captain 
Cook. When surveying along the coast of New Holland in 
1801-1802, Captain Matthew Flinders made the discovery that 
there was a difference in the direction of the magnetic needle 
according as the ship’s head pointed to the east or the west. 
The deviation in wooden ships can be practically obviated, but in 
iron ships it has to be partly allowed for and partly compensa- 
ted. Barlow used a corrective plate of iron to overcome the 
directive action on the compass due to the magnetism of 
wooden vessels. On Professor Airy’s method the permanent 
magnetism of ships is compensated by a steel magnet placed 
at a given distance below the compass. It is, however, liable 
to changes of intensity occasioned by shocks, vibration, unequal 
heating and other causes, a fact which led to the late Dr. 
Scoresby to propose the employment of a compass aloft out of 
the region of the ship’s influence. 
The induced magnetism of ships can be only imperfectly com- 
pensated, since it varies according to the ship’s bearing and as 
she rolls and pitches ; but corrections can be made for the heel- 
ing error. 
In the last January number of the Popular Science Monthly, 
Lieutenant-Commander T. A. Lyons, U.S. N., very graphically 
describes induced magnetism, and its prime importance to navi- 
gation, in his article on the guiding needle of an iron ship. 
Lieutenant Lyons says :— 
“ Let us conceive a metallically pure cylinder of wrought iron 
or cast iron that bas not been hammered, and let us further con- 
ceive it entirely free from magnetism, hold it vertically, and 
