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Page:Popular Science Monthly Volume 61.djvu/192

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POPULAR SCIENCE MONTHLY.

ber of hearings on the subject, published in a pamphlet of 240 pages, and has drawn up a careful report. This report covers familiar ground, but in an unusually clear and straightforward manner. The attitude of Washington, Jefferson and Adams is referred to, and the history of the metric system of weights and measures is briefly reviewed. It is pointed out that the adoption of a decimal system of coinage in the United States was one of the strongest influences leading to the adoption of the metric system by France, and that Great Britain and the United States are practically the only non-metric countries. The weights and measures of Great Britain and the United States are not identical as is generally supposed, and there is no chance whatever that either system will become a universal system. The metric system has become necessary for scientific work; it would decrease the cost and labor of education; it would give unity to our manufactures, and is almost necessary for the extension of our commerce. The admitted expense and trouble involved in the adoption of the system are less, as has been shown in other countries, than is feared, and in any case the longer the adoption is delayed the greater will be the difficulty. The scientific, manufacturing and commercial interests of the country are under great obligations to Mr. John F. Shafroth, who, as chairman of the house committee on coinage, weights and measures, has devoted much careful attention to the subject.

 

THE CAUSES OF VOLCANIC ERUPTIONS.

The recent volcanic outbreaks in the Lesser Antilles have naturally aroused much popular and scientific interest in these geological phenomena and have made a brief statement of current and accepted explanations of them a matter of interest. All these manifestations of heat are derived from the great stores which exist in the interior of the earth. The consideration of them and of the known increase of temperature with depth led earlier geologists to believe that the earth possessed a heated molten core and a cold and relatively thin exterior shell. But as further investigation developed correct conceptions of the rigidity of the globe in resisting strains produced by its rotation and the attraction of other heavenly bodies for its mass; and as the elevating effect upon the fusing points of rocks of an increase of pressure was realized, it was seen that the earth is practically solid clear through and that local reservoirs of molten rock beneath volcanic districts are alone admissible. That local reservoirs exist seems quite well established, and that the rock is sufficiently fluid to enable complex parent magmas to break up into various differential products is the latest result of the investigation of eruptive areas. Volcanoes are moreover arranged along great lines of geological disturbance and fracture as shown in the accompanying illustration. The fractures are naturally the conduits through which the great tension of the internal molten masses is eased by eruptions. The immediate propulsive force which drives the lava to the surface is the next topic of importance which challenges attention. Some geologists believe that the contraction of the globe and the sinking of one side of the great fractures above referred to force out the lava as juice might be squeezed through a rent in an orange. Others, however, attribute the propulsion to the vapors which are held dissolved or occluded in the lava and which are so much in evidence at times of eruption. The frightful explosions and the vast exhibitions of power which they present give much force to this conception. Imagine then a rising tide of lava. As it forces its way through the conduit it spreads earthquake shocks abroad. Reaching the surface its dissolved vapors explode with greater and greater violence and scatter tuffs and breccias over the neighboring country. They may rend