Mr Barlow’s Discoveries on fJie Magnetism Hoi Iron. 361 
iron^ which has hy far the greatest power when cold, has the 
hast of any when heated; and that the cast-iron, which is' the 
hast powerful when cold, the strongest when hot; the increase 
of strength in the latter case being nearly as 3 to 1. 
It was while pursuing these experiments with Mr Charles 
Bonnycastle, that the singular effect to which we have alluded 
presented itself. It was observed by both these gentlemen, that 
between the white heat of the iron (when every species of mag- 
netic action disappears), and the blood-red heat (when the pov/er 
manifests itself so strongly), there was an inteiitnediate action, 
while the iron passed through the shades of bright red and red, 
which attracted the needle the contrary way to that when cold,’ 
or at the blood-red heat ; that is, if the iron and compass are so 
posited that the north end of the needle ds attracted towards the 
iron when cold, the south end will be attracted when the iron is 
red hot, and vice versa ; but as the red changes to the darkest 
shades of blood-red, the usual power of the iron commences, 
and the needle is deflected the contrary way. .Moreover, this 
negative attraction is least in those positions where the natural 
cold attraction is the greatest, and greatest where the latter is 
the least, and greatest qf all in that position where the coM at- 
traction is zero; that is, in the plane of no attraction, provided 
(of course) the needle is sufficiently near to the bar. The bars 
used in these experiments were 25 inches in length. It inch 
square, inclined in the direction of the dipping-needle ; the dis- 
tance varying from 5 to 9 inches; but the nearer to the bar, the 
more obvious are the effects. In some of the experiments referred 
to above, the quantity of the negative attraction exceeded 50°. 
AiiT. .^Practical Rides for the Determination of the 
Radii qf a Douhh Achromatic Object-glass. In a Letter to 
Dr Brewster. By J. F. W. Herschel, Esq. F.R.S.L. & E. 
&c. 
My Dear Sir, 
-A.S you suggest that a popular abstract of the results of my 
paper recently published in the Transactions of the Royal So- 
ciety * on the Aberrations of Compound Lenses and Object- 
* 1821, Part ii. p, 222. 
