140 Mr Christie on the Effects of Temperature 
prism; and these again obtusely acuminated by four planes, 
which are set on the obtuse edges of the first acumination. This 
second acumination appears sometimes to terminate in a line, 
when two opposite planes are much larger than the others. 
2. Rectangular four-sided prism, acuminated by four planes, 
which are set on the lateral planes, and the angles formed by 
the meeting of the acuminating and lateral planes bevelled. 
3. In some crystals the acumination on one extremity is sim- 
ply the acute eight-sided pyramid, while, on the opposite, it is 
the double acumination already mentioned. 
In some specimens, the length of the crystals is three or four 
times greater than the breadth, and in others the crystal is so 
short, that the acuminating planes of the opposite ends meet in 
the lateral edges. The surface of the crystals is smooth and 
shining, and they range from transparent to feebly translucent. 
They are generally small, many of them not exceeding in size 
the head of the smallest pin. The largest I detached is about 
j^ths of an inch in length. 
Specific gravity 4.409, Dr Turner. 
It is right to add, that the chlorite containing the zircon is 
associated with magnetic iron-ore, talc-slate, and serpentine. 
Art. XXIV. — On the Effects of Temperature on the Intensity 
of Magnetic Forces ; and on the Diurnal Variation of the 
Terrestrial Magnetic Intensity. By S. H. Christie, Esq. 
M. A. of the Royal Military Academy. 
In the last Number of this Journal, we laid before our readers 
an interesting extract from the memoir of Mr Christie on Mag- 
netism, &c. not then published. This important memoir ha- 
ving just appeared in the Philosophical Transactions , Part I. 
for 1825, we shall now state some of the facts and views which 
it contains. It commences as follows. 
ct In the paper on the diurnal deviations of the horizontal 
needle when under the influence of magnets, which the Presi- 
dent did me the honour to present, I stated that these deviations 
were partly the effects of changes that took place in the tempe- 
rature of the magnets ; and that although the conclusions which 
