By E. D. COPE.--Numerous illustrations.

VI. NATURAL HISTORY.--The Diamond Fields of South Africa.

Sponges at the Bahamas.

Testing Fish Ova for Impregnation.

VII. MISCELLANEOUS.--The Production of Fire. 4 figures.

St. Blaise.--The winner of the Derby. 1 illustration.

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IMPROVED DYNAMO MACHINE.


The continuous current and the alternating current generators invented
by Dr. J. Hopkinson and Dr. Alexander Muirhead are peculiarly
interesting as being probably the first in which the bobbins of the
armature were wound with copper ribbon and arranged on a disk armature
much in the same way as was afterward done by Sir William Thomson and by
Mr. Ferranti. In the Muirhead-Hopkinson machine the armature coils are
attached to a soft iron ring, whereas in the Ferranti the iron core is
dispensed with, and a gain of lightness in the armature or rotating part
effected; this advantage is of considerable importance, though Messrs.
Hopkinson and Muirhead can of course reduce the weight of this iron core