Page:Popular Science Monthly Volume 20.djvu/689

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were this isthmus to be submerged, the great body of heated water, that at present finds its way north through the Straits of Florida, might flow directly into the Pacific Ocean. There is no evidence, however, to show that the barrier of Darien has been thus submerged within any recent geological period, and consequently the great changes of climate referred to above could not have been brought about in this way. They were most probably induced, as Dr. Croll has shown, by a remarkable series of physical mutations, which accompanied and were caused by the last great increase in the eccentricity of the earth's orbit. As the next period of high eccentricity will not occur for many thousands of years, we need have no apprehension that we shall be deprived of the genial influence of our friendly Gulf Stream in the immediate future. And when we reflect upon the magnitude of the submergence of Central America which would be required to divert that stream into the Pacific, we may be equally reassured that we are not likely to suffer from its loss in our day, for, before it could be sensibly diminished in volume by this means, the Isthmus of Darien would need to be sunk to a depth of not less than 800 feet, and perhaps even 1,000 feet. We may therefore contemplate with perfect composure the formation of any number of canals across that isthmus. The great work of the famous French engineer will have as much effect upon the Gulf Stream and the climate of North western Europe as the emptying of a teapotful of boiling water into the Arctic Ocean would have in raising the annual temperature in Greenland.—British Trade Journal.


By W. H. PREECE, F. R. S.

I OUGHT to commence, if I carried out the customary practice, by addressing the audience as ladies and gentlemen, but to-night I much prefer commencing with boys and girls, because the two lectures that I am about to deliver to you are really intended to be addressed to the youngest members of the society. I would first direct your attention to the fact that this hall is illuminated by electricity, and, although I do not intend in any way to refer to the electric light to-night, it is my intention, step by step, to lead you up to it, so that, when we part this day week, I hope and trust to be able to let you depart with a thorough and complete knowledge how electricity works and how it produces these marvelous effects. To accomplish this object there are two things most essential: the first is, that I, your lecturer, should be perfectly clear in my definitions, and simple in the

  1. Lecture delivered before the Society of Arts, December 28, 1881, and reprinted from the journal of the society.