Case II.-The Determination, experimentally, in Magnets of
different lengths, of the relation of their respective deviating
effects (at distances from the Compass proportional to their
several lengths) to their actual directive powers

Case III. The comparative Capacities for Magnetism of equi-
valent, or similarly proportioned, Bars of Steel of different
lengths, as Determined both by the method of Deviations and
that of Torsion

-

Sect. I. As to the proportional Powers of Magnets of different
lengths, but in all other respects, both as to dimensions in
breadth and thickness, and in quality and hardness of Steel,
the same

PAGE

292

295

310

- 313

Sect. II.-As to the proportional Powers of Magnets combined
as in the figures (plate iv.), compared with those in a single
pile, or fasciculus
Sect. III. As to the relation of the Powers of Bars of twelve
and of those of eighteen inches when combined so as to con-
stitute masses of the same form, dimensions, and weight - 315
Sect. IV. As to the relative Powers of Combinations of twelve
or eighteen inch bars, each kind by itself, and those of equal
masses and dimensions composed of the two kinds, intermixed 316
Sect. V. As to the relative Powers of various Combinations of
Bars of the same description (except as to the length of the
integrant bars) in proportional masses
. 317
Sect. VI.-As to the extent to which small Magnets, made
quite hard, may be advantageously combined, with a view to
the formation of larger artificial Magnets than have yet been
constructed

GENERAL RESULTS

- 319

CHAPTER VIII.

OF THE MAGNETICAL POWERS, RECEPTIVE AND PERMA-
NENT, OF CAST IRON, BOTH IN SEPARATE BARS
OR PLATES, AND IN VARIOUS COMBINATIONS

PLATES

OF

RESULTS

- 330
- 344

CHAPTER IX.

ON THE MEASURE OF PERMANENCY OF THE ENERGY
IN STRAIGHT-BAR MAGNETS, BOTH SINGLE AND
COMPOUND, AND OF DIFFERENT DEGREES OF HARD-
NESS, AS SELF-SUSTAINED, OR AS INFLUENCED ONLY
BY TERRESTRIAL MAGNETISM

AS TO THE POWERS OF COMBINATIONS OF MAGNETISED PLATES OF TEMPERED STEEL, IN CONTACT.

THIS important object of inquiry-as affecting both the directive power of compass needles and the energy of compound magnets--has been pursued to a much greater extent, and with a much larger variety and assortment of plates and bars, than it may here be necessary, or indeed useful, to describe.

In the whole, the powers in combination of about forty sets of plates or bars, have been carefully tried, in their respective series of from 2 to 192 together, the total amount of magnetised pieces of steel, subjected to experiment, being scarcely less than from seven to eight hundred. And in several of the sets of bars or plates, the powers in combination were likewise

Н

on the deviations produced by the numerous magnetical plates and bars subjected to experiment,--each observation requiring the needle of a five-inch compass, after being disturbed by the influence of the magnet to be tested, to attain a stationary position, and the angle of deviation from the magnetical meridian, to be read off to within a minute or two of a degree. And besides this labour by the method of deviations, a large number of magnetical bars of the horse-shoe form, etc., had to be otherwise tested; and the results obtained by the different modes of experiment required to be tabulated, in many cases at a considerable addition of trouble, for reducing the observations, and for obtaining the exact measures and weights of the bars or plates employed.

Similar investigations, as some of these here described, have, as is well known, been before made, and analogous results, in certain cases, obtained. The very extensive range of inquiry here pursued, however, with a constant adherence to the same modes of experimenting and testing, will, the Author hopes, not only excuse his having gone over some ground previously examined; but will yield a measure of newness -by the unity of method and ampleness of investigation in the vast varieties of mass, form, quality, temper, and denomination of the magnets made use of-to the researches themselves.

Besides the experimental inquiries of the first and second Parts of these Magnetical Investigations, the Author has always contemplated, if leisure and health should, in Providence, be yielded him, to extend the work to two or three additional parts, embracing,-Practical Magnetics, with the application of these Investigations to the improvement of Magnetical Instruments and Apparatus; various original illustrations of, and experiments on, Magnetical Principles, and Phenomena, etc., etc.

It may be proper to state, however, that the two Parts of the work now before the public have been so adjusted as to be complete in themselves, the last sheet of Part I., having been cancelled, and the substituted sheet G placed at the conclusion of this Part, in order that the connection betwixt the two portions might be more perfect.

The Author has only, in conclusion, to add, that whatever benefit to science his labours might be calculated to yield, or to his country,for aiding the improvement of an instrument heretofore so exceedingly defective and uncertain as the sea-compass,-such benefit he is most desirous should be fully realized, and he has taken some pains to render that desire effective. In order to this, the Author has communicated, without reserve, to Messrs. Stubs (of the firm

"Peter Stubs "), manufacturers in steel, of Warrington, the principal practical modes of constructing, magnetizing and testing the various kinds of plates and bars described in this work, or designed for being employed in magnetical apparatus. The employment by Messrs. Stubs of first-rate workmen in all the departments requisite for the construction of magnetical instruments, and especially for hardening or tempering; their extensive engagement as manufacturers of steel; with the deserved celebrity which their files and tools have gained for their firm, not only in England, but in many parts of the world abroad,-will at once enable them to undertake the kind of work referred to with peculiar advantages, and be a guarantee to the Author and to the Public, of its being faithfully and efficiently done.

BRADFORD, YORKSHIRE,

November 7th, 1843.