584 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 ex- 

 treme. The faunas of the Cambrian, and especially of the Lower Si- 

 lurian 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. 



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T 



MAGNETO-ELECTKIC ILLUMINATION. 



By WILLIAM CEOOKES, F. E. S. 



HE progress made in electric illumination during its advance 

 -J- 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 cur- 

 rents for industrial purposes have been their almost instantaneous 

 character and the rapid alternation in their direction. The instru- 

 mental 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 re- 

 versed 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-hou6es, the first arma- 

 ture is made to revolve about 2,500 times a minute, generating 5,000 



