426 Scientific Intelligence. 
ties. In order to form tangent galvanometers of great sensibility, 
Gaugain proposes to place the centre of the needle at the apex of a 
right cone, the angle of which can easily be determined for a needle 
of any given length. If the surface of the cone be wound with wire 
covered with silk, each spire will produce a deviation, the tangent of 
which will measure the intensity of the current, and the conical multi 
plier thus formed will of course enjoy the same property. The results 
obtained by Gaugain have been submitted to an analytical investigation 
by Bravais, who has found that they are readily deduced from Am- 
pere’s theory, and that the error committed in supposing an exact pro- 
portionality is very small when the radius of the circle is not less than 
three times the length of the needle. In the case of a needle the 
length of which is one-sixth of the radius this error would be about 
Tz50 of the whole intensity.—Compies Rendus, xxxvi, 191, Jan. 24, 
2. ‘On the double refraction produced by compression in Crystals be- 
longing to the regular syslem.—WxrtHEim has communicated the re- 
apis of a second investigation* of this subject, which are as follows: 
Every miners! species belonging to the regular system has @ 
en co-efficient of elasticity determinable with sufficient accuracy 
y means of the an thle tone which is given by plates vibrating 
irgnes nee’ with both ends free. 
2.) Crystals which exhibit only the faces of the cube, behave like 
homogeneous bodies toward external forces. Under equal circumstan- 
the extraordinary and the ordinay ray, whatever be the direction m 
which the force acts, provided only that it is always perpendicular io 
the surfaces of the crystal. 
n the case of rock salt and fluor spar, which crystallize in 
cubes, the difference of path for an equal linear compression is a 
the same as in sealed species of glass; the aaah doubly refracting 
power is also the same. 
.) Alum, abich crys stallizes in cubo-oct tabield eh does not behave 
like an optically homogeneous body, although its elasticity is ‘equal in 
all directions. The forces which must be applied to produce an equal 
difference of path often vary as 1:4 according to the direction in 
which they a 
(5.) It has already been shown that in alum the optical and mechan- 
ical axes do not coincide ; this acdc takes place as if the posi- 
tion of the optic axes had been traced beforehand within the erystal ; 
it is exhibited hc oa right or pee the left of the observer accor- 
ing the other of me two faces traversed by the ray is 
~ 
a 
ee 
a This displacement is the more considerable in — perpen- 
dicular t oO Hs aces of the cube, the less regularly the’ se faces are 
itis zero or almost zero in crystals with the prasoe faces 
the cube, but but it increases in proportion as these faces differ from the 
form of a square, and it is ofien from 20° to 25° when, in consequence 
of of these of formation which we usually consider 28 
* This Journal, xiii, 411, May, 1852. 
