MAGNETIC SURVEY OF TASMANIA. 83 
site. ‘Phe declination of Hobart (1901), as found by. inter- 
polation from determinations elsewhere, is about 9° 59! EF. 
The changes in the declination of Hebart since 1842 are 
shown in Plate I. 
At each of the eleven selected sites absolute determi- 
nations of the magnetic dip, declination, and horizontal com- 
ponent of the earth’s magnetic force were made. It was 
also necessary for us to determine in each the latitude and 
the longitude relative to Hobart of the site. At the exact 
spot where the theodolite was set up for astronomical work 
a mark has been left, and an exact description of the locality 
of each mark is given below. [The magnetic observations 
were not taken on this exact spct.| Where possible, 
the theodolite was set up so that a small Muntz 
metal cylinder, about 4 inches leng and 1 inch in diameter, 
could be let inte the solid rock. When this could not be 
done, a hole some 3 feet deep was dug; a good base of stones 
was put down, and well driven in; on the top of this was 
placed a block of cement, about 10 inches or a foot square 
and 6 inches deep, and in this the metal plug was inserted, 
ihe top of the plug being at a depth of about 1 foot from the 
surface of the ground. The hole was then filled up. The 
end of the cylinder was engraved “M.S., 1901,” and the 
mark will below be called \the M.S. mark. From 
each ‘site a reading was taken of some well defined distant 
object, which could, from the description ziver by us be 
picked up by a surveyor, and the azimuth of this mark 
has been determined relative to the meridian passing 
through the site. At ali stations, where practicable, azimuth 
readings were taken of well-defined points on prominent 
natural objects, such as mountains, in the hops that the 
information so collected might at some future time be of 
use in the compilation of the map of Tasmania. 
The actual magnetic results obtained durmg tke survey 
are best studied on the maps shown. The Imes of equal 
dip, equai horizontal intensity, and equal declination have 
been plotted for us by Mr. F. M. Young, B.A., one of the 
vice-presidents of this section. 
From the map it will be seen that, excluding the South- 
port obseryations for the moment, the magnetic dip varies 
between 89° 56’ 43// at Circular Head and 71° 49! 44°! at 
Port Davey, and that the lines of equal dip show ihemseives 
as roughly parallel. The horizontal intensity is greatest 
ai Springfield, near Scottedale, where its value is «2140, and 
least at Port Davey, where it falls to -1975, and 
the lines of equal ‘horizontal intensity retain _through- 
out the Tslana a roughly parallel course. The dis 
