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Messrs. Editors.

EFERRING to Dr. Garretson's communication in the February number of "The Popular Science Monthly," relating to rock-vibration in the Patapsco Valley, I would state that a like phenomenon has often been manifested at Cuyahoga Falls, Ohio, and has been the subject of much curious speculation on the part of the inhabitants of that village. It has been supposed that the vibrations were caused by the water flowing over the river-dams, but the mystery has been, why the effect should be produced at such great and irregular intervals. Sometimes the vibrations do not occur with noticeable force within intervals of less than one or more years, and yet the quantity of water flowing over the dam increases and diminishes almost continually within its maximum and minimum limits. When the rock-vibrations occur they produce an intermittent rattling of loose window-sashes and jarring of buildings. The Cuyahoga River, which runs through the village, is about fifty or sixty feet wide at the first dam. The height of the dam maybe twelve feet. The back water extends about two miles, the banks being sloping and of moderate height. Below the first dam the river runs through a rapidly descending rocky gorge, there being four or five dams within a distance of about half a mile.

Messrs. Editors.

to ask whether the following definitions, distinguishing the three states of matter, have ever been given to the public. I have not found them in any text-book. Also, whether the test given herewith, distinguishing the two kinds of forces, has ever been formally stated in any scientific work. If not, I think it would be well that they should both appear in "The Popular Science Monthly." I submit them to your consideration: Definitions distinguishing the three states of matter.—A solid is matter whose cohesion is greater than gravity upon its particles. A liquid is matter whose gravity is greater than the cohesion of its particles. A gas is matter, the repellent force (or heat) of whose particles is greater than gravity and cohesion combined. Test distinguishing the two kinds of force.—All forces are divided into two kinds, attractive forces and motive forces, or attractions and motions, and may thus be distinguished: All motive forces can be insulated; no attractive force can be insulated. Had this test been understood in the days of Newton, it might have saved the scientific world immense experimental labor.



HERE are many who will regret to learn that this great work has been brought to a close. It has not been carried, as far as its projector originally intended, but still we can not say that it has stopped prematurely or remains merely as a fragment. On the contrary, it is substantially accomplished. It was a large enterprise, broadly conceived by Spencer twenty-five years ago, as a comprehensive basis on which to build the superstructure of sociological principles. The undertaking involved a carefully digested method of collecting and classifying all the main orders of facts which represent the constitution and characteristics of different human societies, in a form suitable for convenient reference and ready comparison; and it was on a scale that implied the co-operation of several scholars working through many years to execute it. It was inevitable that this task should be of gigantic proportions and involve enormous labor, because social generalizations, to have value, must be based upon that which is both