Natural Philosophy. — Magnetism. ITT 
paper contains no allusion whatever to the experiment of Dr 
Wollaston, which has been truly characterised as an elegant one 
both by M. Biot and Mr Brande. If Mr Murray, after examining 
the works referred to, shall consider his historical statement re- 
specting the aethrioscope as correct, we shall willingly retain it, 
as it is not our business to decide for others. 
OPTICS. 
6. Remarhable Dichroism qf^ Tourmaline.— -N very interest- 
ing specimen of dichroitic tourmaline in the cabinet of Mr 
Allan, exhibits the most singular contrast of colours that I have 
yet found in any substance. The plate is cut perpendicular to 
the axis of double refraction, and also to the axis of the prism. 
In the direction of the axis the colour is a deep and brilliant 
blue, while in a direction at right angles to the axis, the colour 
is a very pale red approaching to pink. — D. B. 
MAGNETISM. 
T. On the best kind of Steel and Form for a Compass-Needle. 
— In the Bakerian Lecture “ on the best kind of steel and form 
for a compass-needle,” by Captain Kater, published in the Phil. 
Trans. 1821, Part I. the following results are given . — “ 1. That 
the best materialfor compass-needles is clock-spring; butcare must 
be taken in forming the needle to expose it as seldom as possible 
to heat, otherwise its capability of receiving magnetism will 
be much diminished. — 2. That the best form for a compass- 
needle is the pierced rhombus^ in the proportion of about five 
inches in length to two inches in width, this form being suscep- 
tible of the greatest directive force. — 3. ^I'hat the best mode of 
tempering a compass-needle is, first to harden it at a red heat, 
and then to soften it from the middle to about an inch from each 
extremity, by exposing it to a heat sufficient to cause the blue 
colour which arises again to disappear. — 4. That in the same 
plate of steel of the size of a few square inches only, portions 
are found varying considerably in their capability of receiving 
magnetism, though not apparently differing in any other respect. 
5. lhat polishing the needle has no effect on its magnetism. 
— 6. That the best mode of communicating magnetism to a 
needle, appears to be by placing it in the magnetic meridian, 
joining the opposite poles of a pair of bar magnets (the magnets 
VOL. Vf, NO. 11 . JANUARY 1822 , 
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