TERRESTRIAL MAGNETISM.—MURPHY. 295 
the present year. I am not aware of any record being kept at 
Halifax, N.S. I cannot speak with any degree of reliability 
respecting the dipping needle here. The declination was about 
20.50 degrees west of north, and I am informed by the officers of 
the Crown Land Department that there is an annual variation 
of about three minutes per year. 
If a magnetic needle be pivoted so as to confine its motion to 
the horizontal plane as an ordinary compass needle, the north 
end of such a needle when at rest will deviate in the azimuth to 
the west of true north by the amount of this magnetic declina- 
tion or variation of the compass, which varies with locality. 
For any place situate on one of the magnetic lines on the map or 
chart the value of the magnetic declination will be exactly that 
of the line, and for places situate between the two lines propor- 
tion will be made between the values of the adjacent lines. Such 
values are generally made to apply to the year for which the 
map or chart has been issued. The values shewn by the lines on 
the English chart or map are for the year 1889, but to obtain 
values for any other epoch it must be understood that the mag- 
netic declination over the area included in that map diminishes 
yearly by about seven minutes of an arc. Thus mean values for 
any place and for any time within a few years may be found. 
Values of magnetic declination obtained in the way described 
will, it is presumed, serve for ail purposes of mine surveying. It 
may, however, be further mentioned that the needle is subject 
also to diurnal variation, its north end being drawn most to the 
west at about two hours p. m., and most to the east during the 
night or early morning, occupying its mean position about 10 
hours a. m., and 6 hours p.m. The diurnal variation is greater 
in summer than in winter, but the needle seldom deviates from 
its mean position more than from five to ten minutes of an are, 
excepting during what are called “ magnetic storms.” During 
the year 1888 there were very few occasations of much change 
recorded at London, so we seem to be passing through a quiet 
period. 
The English map produced has been tested by comparing 
results obtained at Greenwich, Kew, Stoneyhurst and Falmouth, 
