Description of the Solar Index. 301 
strata of the globe, they may all, together with the phenomena 
of volcanic action, earthquakes, metamorphic action, the trans- 
port of the materials and the deposition of the sedimentary rocks, 
the great currents of the ocean, the distribution of fossiliferous 
organic life, the various derangements of the strata, and most of 
the phenomena of geology, be referred to the secULAR REFRIGER- 
ATION OF THE EARTH, combined with the effects of gravitation, 
inertia, and the rotation of the earth on its axis. 
The effects of these causes can be traced in imperishable 
characters in the solid strata of the globe through unknown ages 
of past duration, and the same causes may be expected to con- 
tinue to act through all future time. 
Art. VI.—Description of the Solar Index, a new Magnetical 
Instrument; by Marsuauu Conant. 
‘Tue ‘Solar Index” is the name I give to an instrument which 
I have recently contrived, the object of which is when attached 
to the surveyor’s compass to show at any place the variation of 
the magnetic from the true meridian ; consequently enabling the 
surveyor or engineer with very little trouble to take his courses 
from the true north and south. 
An instrument for this purpose, of simple construction, appears 
to have been for a long time a desideratum. And several forms 
at different times and by different individuals have been tried and 
abandoned, or obtained but very limited use. The simplicity of 
this, and the mode of using it, as well as the accuracy of the re- 
sults it affords, seem to indicate a greater degree of utility; and 
induce me to send a description of the instrument, accompanied 
by its delineation, for publication in the American Journal of 
Science ; adding that I do not design to restrict the manufacture 
and sale of it by means of a patent, but on the contrary wish it 
to be considered a free contribution to the arts. 
The material of the instrument, except one or two pieces, is 
brass. A, is its base, which is fixed to the base of the compass by 
a single screw from underneath, having a milled head and of a 
size equal to those by which the sight vanes are usually attached 
to the compass. One edge of this base is concave, so as to fit the 
circular box of the compass on the outside. B, is a horizontal 
Vol. xi1x, No. 2.—July-Sept. 1845. 39 
