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THE experiments described in this second Part of Magnetical Investigations," were, to a very considerable extent, made some years ago. They have been carried forward, however, at intervals, as the Author's arduous professional duties would admit, to the present time; and the publication of the results obtained has been deferred until all the objects of inquiry, belonging to the department of experimental research, which were originally contemplated, or have been suggested in the progress of the Investigations, have been completed.

It is hardly needful to say that the researches have been of a very elaborate nature and extent. The Tables given in this part of the work alone are the result of above four thousand observations

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.

November 7th, 1843.

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