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THE POPULAR SCIENCE MONTHLY.

of social life often makes it seem that the closing of the school is indispensable to the checking of the disease. The necessity of such an extreme measure may, however, be nearly always prevented by the exercise of proper foresight. The teacher should be watchful of absences and their causes, and should give notice, when infectious disease is in question, to the proper officers. The sick pupil should then be isolated from the well ones, and his home and family surrounded with the most rigorous sanitary precautions for not less than eight weeks. Dr. David Page, sanitary officer of Westmoreland, England, has always avoided the necessity of extreme measures by adhering to these principles. The school should be closed only when it has obviously become a starting-point of infection, or when the control specified above can not be exercised. The term of suspension must be determined by circumstances, and can not be previously regulated, but a premature reopening should be avoided; and the reopening should be preceded by a thorough disinfection, by fumigation with sulphur and washing the walls with lime and the wood-work with soap and carbolic acid. The continuance of day schools during the prevalence of scarlatina is justified, says Dr. Page, when the children would be otherwise exposed to much risk in playing about their doors with children of infected families, and with those barely recovered from illness. Under such circumstances, always provided that due supervision over infected families is maintained, a child runs less risk in regular attendance at school. But in scattered country districts, where the children coming from all points are brought together only during school hours, the breaking up of the school is the best and safest course.

 

Earth-Tremors.—The committee appointed by the British Association, two or three years ago, to measure the lunar disturbance of gravity, have met with unexpected difficulties in the accomplishment of their task, and have substantially given it up as for the present unattainable. The Messrs. Darwin, who undertook the observations at Cambridge, found that, as soon as they had made their instrument sensitive enough to record the lunar disturbances, they had to deal with other disturbances, "so incessant and so lawless that the steady march of the lunar swing was utterly overborne and lost." The earth was never really still. It quivered and throbbed and warped and bent under the pendulum night and day, and even, as it seemed, in the absence of all merely local agencies that could be detected. A situation at the bottom of a deep mine was then suggested, but with no better success. The earth yields there under the operation of deep-reaching causes that can not be got rid of, and which produce effects of the same order of magnitude as the direct effect of the moon, and are at present inextricably entangled with it. These causes are the varying mass of the air, that shifts and changes according to the indications of the barometer, and the varying mass of the water on the shores, that shifts and changes with the tides. It is easy enough to believe that, when a mountain-mass is set down upon the earth, the crust must yield and a depression form at the spot upon which the excess of weight is placed. "But it was probably never imagined till now that, when the barometer rises an inch over a land area like that of Australia, the increased load of air sinks the entire continent two or three inches below the normal level. Over a like sea area the water surface may be depressed a foot or more. Thus, as the mass of air sweeps in wind or creeps by slower convection from place to place, the yielding earth sways up and down beneath its weight"; a depression is formed, toward the center of which the surface slopes from all sides, and the plumb-line ceases to be perpendicular to the surface. The mass of air which hovers over the spot also acts like a mountain and draws the pendulum toward it; the two effects are superimposed, and the apparent displacement of the vertical is exaggerated. The two influences always act together, and are proportional; and this twofold deviation is of the same order as that which the moon produces, but is perpetually varying and incalculable. It therefore vitiates all pendulum observations. The tides exercise a similar power, depressing the shore at the flood and allowing it to rise at the ebb. The advance and retreat of the water will also tell on the plummet by mere attraction. The lead will seem to be pulled