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XXVII. On the Injluence of Screens in arresting the Progress of Magnetic 
Action. By William Snow Harris, Esq. F.R.S. 
Read, June 16, 1831. 
i. Whilst engaged in some experiments with a thin plate of iron, em- 
ployed as a screen to intercept the action of a magnet, the following curious 
fact presented itself to my notice. Although the single plate of iron, which 
was about the T Vth of an inch in thickness, very effectually screened the action 
of a revolving magnet on a disc of copper, the magnet and disc being placed 
at a certain distance from each other ; yet the same effect did not follow when 
the disc acted on by the magnet was also of iron : in the latter case it was 
found requisite to increase the quantity of intervening iron very considerably. 
This was done by piling several plates similar to the first, one on the other, by 
which the quantity of iron requisite to intercept completely the action of the 
magnet could be in some measure estimated. 
From this I have been led to some new inquiries concerning the screen- 
ing influence of substances generally, which, I trust, may possess some claim 
to the consideration of persons engaged in scientific pursuits. I have there- 
fore to beg the favour of being allowed to lay them before the Royal Society. 
2. Having carefully repeated the experiments from which the result just 
mentioned was obtained, I subsequently endeavoured to determine if the 
screening influence of the iron plates depended actually on the mass, or was 
otherwise confined to the plates immediately on the surface. With this view 
the interior ones were removed, and a ring of wood equal in thickness sub- 
stituted in their place ; the position of the upper and under plate therefore 
remained unchanged. 
In this instance, however, the magnetic action was no longer effectually 
intercepted, nor could it be completely shut out, except by restoring all the 
interior plates as before ; that is to say, two plates above and two below were 
insufficient, and so on. 
3 s 2 
