TERRESTRIAL MAGNETISM.—MURPHY. 309 
needle to point in its natural direction however the ship may 
head: such a contrivance is known as a compensating binnacle. 
In considerable changes of magnetic latitude the magnets 
have to be slightly moved to counterbalance the altered condi- 
tion of the deviations, and sometimes, also, to compensate for a 
partial loss of power in the magnets themselves. 
The more immediate object of this paper is to point ont what 
occurs to the author respecting the inffuence of terrestrial mag- 
netism on iron bridges, iron rails, locomotive machinery, and 
such other structures and machines as undergo great stress, 
through blows, shocks, or violent contacts, in latitudes where 
terrestrial induction is of great moment. I would also wish to 
make some reference to the frequent and fitful changes exhibited 
at the different magnetic observatories, as that they might in a 
great degree be attributed to thermal changes in the body of the 
great magnet, the earth itself ; but as my time is now exhausted, 
I should prefer deferring any further remarks to a future 
evening. The plates referred to in this paper will appear in 
that which is forthcoming. 
