10 REPORT—1852. 
ratus and a rotating machine, I obtained very distinct phenomena of induction by the 
action of a bundle of varnished bismuth or copper wires. I have already observed 
that the experiment can be decisive only when one acts on the electro-magnet with 
a sufficient quantity of the diamagnetic substance. In fact, it seems reasonable to 
admit that the effects of induction, magnetic or diamagnetic, ought to be in propor- 
tion with the corresponding effects of attraction or repulsion. Now I am not very 
far from the truth in admitting that 1 gramme of bismuth is repelled by an electro- 
magnet with the same force as 11 miliigrammes of colcothar is attracted by the 
same magnet; that 1 gramme of sugar or stearic acid makes equilibrium to 5 or 6 
milligrammes of colcothar, and 1 gramme of sulphur to 2 or 3 milligrammes of col- 
cothar. I have already said that the inductive action of 500 grammes of colcothar 
gives me 10° to 15° of induced current: therefore, to obtain a similar effect by bismuth 
(if diamagnetic polarity exists), one must employ a quantity of that metal, which is 
at least a hundred times 500 grammes, or 50 kilogrammes. By similar reasoning one 
sees what an enormous quantity of phosphorus, sugar and sulphur would have to be 
employed in order to obtain a sensible inductive effect, and how far we have hitherto 
been from employing the necessary quantity. It is only by the method of rotation 
of inductive bodies in presence of the electro-magnet, that one can obtain sensible 
effects from small quantities of diamagnetic substances. Using the method of 
rotation, as Verdet has done, with an electro-magnet and inductive coil more powerful 
than any which have been hitherto employed, I have succeeded in obtaining distinct 
signs of induction from a mass composed of fragments of varnished bismuth. 1 con- 
tinue to vary and extend my experiments in this way; therefore, for the present, 
though I should be grateful to you if you would communicate this note to the British 
Association, I wish to be able to arrange these researches myself before publishing 
them. 
On placing Compasses on Board Iron Ships. By Captain E. J. Jounson, 
RN. F.R.S., Superintendent of the Compass Admiralty Department of 
the Royal Navy. (In a Letter to the President.) 
It was my intention to have been present at the Meeting of the British Association 
at Belfast, but I have been prevented by my official duties on board some of H.M. 
steam-ships which could not be delayed. One of these was the iron steam-vessel 
‘¢ Trident,” and I think it worth while to notice to you a circumstance which oc- 
curred relating to the compass observations. 
As a member of the Compass Committee, you are aware that the system adopted 
in H.M. service on board iron ships, is to elevate the compass considerably—to 
ascertain the deviations and allow for them, and to persevere in a continual series 
of observations to ascertain the change of deviation according to the change of the 
ship’s geographical position, as described in the “‘ Practical Rules’? which have been 
issued to all H.M. ships sizice 1842 ;—the said plan being considered safer than the 
application of iron or magnets for the reduction of the amount of deviation, 
In placing the compasses of H.M. ships, I have, of course, adhered to the recom- 
mendations of the Committee, taking care by a few preliminary observations to fix 
upon a position where the deviations were lessened ; but the circumstance to which 
I wish to draw your attention at present is this :—While the ‘* Trident’’ was in the 
basin at Woolwich, it occurred to me to try whether a position could be discovered 
where the influences of the ship’s iron upon the compass were so equalized as to 
render the amount of deviation so small as to be of na practical importance. 
The correct magnetic direction of the ship’s head having been determined by a 
compass on the shore, and that proving to be near to one of the points of maximum 
deviation (the standard compass on the quarter-deck there indicating 20° westerly 
deviation), I moved the standard compass several feet further forward in the centre 
line of the ship, and there found the westerly deviation increased to 29°. I now 
commenced to move the compass aft 6 or 7 feet at a time, observing the deviation 
at each position, and found the weséerly deviation decreased; and on placing the 
tripod of the compass direetly over the rudder-head, easterly deviation was produced ; 
and hence it followed that there must be a position somewhere between the two last 

