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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 i,hen be isolated from the 

 well ones, and his home and family sur- 

 rounded with the most rigorous sanitary 

 precautions for not less than eight weeks. 

 Dr. David Page, sanitary officer of West- 

 moreland, 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 con- 

 trol 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 re- 

 covered from illness. Under such circum- 

 stances, always provided that due supervis- 

 ion 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 ap- 

 pointed by the British Association, two or 

 three years ago, to measure the lunar dis- 

 turbance of gravity, have met with unex- 

 pected difficulties in the accomplishment 

 of their task, and have substantially given 

 it up as for the present unattainable. The 

 Messrs. Darwin, who undertook the obser- 

 vations 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 steadv 

 march of the lunar swing was utterly over- 

 borne and lost." The earth was never really 

 still. It epiivered and throbbed and warped 

 and bent under the pendulum night and dav, 

 and even, as it seemed, in the absence of all 

 merely local agencies that could be detect- 

 ed." A situation at the bottom of a deep 

 mine was then suggested, but with no bet- 

 ter 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 di- 

 rect 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 mounta'm-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 convec- 

 tion 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 hov- 

 ers over the spot also acts like a moun- 

 tain and draws the pendulum toward it ; 

 the two effects are superimposed, and the 

 apparent displacement of the vertical is ex- 

 aggerated. 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 per- 

 petually varying and incalculable. It there- 

 fore 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 



