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Page:Popular Science Monthly Volume 3.djvu/600

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THE POPULAR SCIENCE MONTHLY.

of older rocks wherewith to form them. Probably, like the American Laurentian rocks, that old land lay in the north, but whether or not, of this at all events I have more than a suspicion, that the red, so called Cambrian, beds at the base of the Lower Silurian series indicate the last relics of the fresh waters of that lost continent, sparingly interstratified with gray marine beds, in which a few trilobites and other sea-forms have been found. Going back in time beyond this, all reasoning or detailed geological history becomes vague in the extreme. The faunas of the Cambrian, and especially of the Lower Silurian rocks, from their abundance and variety show that they are far removed from the beginning of life. Looking to the vanishing point in the past and the unknown future, well might Hutton declare that in all that the known rocks tell us "we find no vestige of a beginning—no trace of an end."—Contemporary Review.

 

MAGNETO-ELECTRIC ILLUMINATION.
By WILLIAM CROOKES, F. R. S.

THE progress made in electric illumination during its advance toward perfection has been several times recorded in the pages of this journal. In our first number, published nearly ten years ago, Dr. J. H. Gladstone gave a history of the early difficulties attending the introduction of the magneto-electric machine as a light-generator for light-house illumination. Two years subsequently, the present writer described Wilde's magneto-electric machine, and, after a further lapse of years, during which time no very important improvement in the industrial application of magneto-electricity has been recorded, another step in advance has been made which calls for detailed notice.

The chief difficulties in the employment of magneto-electric currents for industrial purposes have been their almost instantaneous character and the rapid alternation in their direction. The instrumental means necessary to seize hold of these rapidly-alternating waves, and convert them into a more or less continuous stream of force flowing in one direction, are necessarily of a delicate character, and are easily put out of adjustment. This is easily understood when it is remembered that, in the machine first tried by Mr. Holmes, the rubbing surfaces were worn away in ten or twenty minutes. The Berlioz machine required for its maximum of intensity 350 or 400 revolutions per minute, and the direction of the current is then reversed nearly 6,000 times per minute; here, however, the alternate currents are not brought into one. In the machine made by Mr. Wilde for the Commissioners of Northern Light-houses, the first armature is made to revolve about 2,500 times a minute, generating 5,000