Page:Popular Science Monthly Volume 6.djvu/651

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falls 10° in a few minutes, but observations show that there was no air-current from the north. These sudden gusts of cold must descend from the higher atmospheric regions.


School Hygiene.—Dr. Richard Liebreich, the eminent oculist, read a paper on "School Hygiene" at a recent meeting of the London Social Science Association. He spoke of the influence of the posture of children during school-time, restricting himself to the discussion of two points, namely, short-sightedness and lateral curvature of the spine. Short-sightedness, he said, is produced by the lengthening of the anteroposterior axis of the eye, by the increased tension of the apparatus with which we adapt our eyes to different distances. The tension is stronger in proportion as the distance is shorter to which the eye is adjusted. If this power is made use of for adjusting the eye continually to a much shorter distance than would be required; i. e., if, in reading or writing, the eyes, instead of at twelve to fifteen inches, are kept at four to six inches' distance from the book, the sclerotic, or membrane which keeps the globe of the eye in shape, giving way by degrees to the pressure, gradually extends antero-posteriorly. Thus the eye becomes oval, and the retina is somewhat removed from the optic media, from the cornea, and the lens. The retina then only receives the images of near objects; distant objects appear undefined, and can only be seen by the aid of concave glasses.

The abnormal posture of children during school-time, and especially while writing, is productive of spinal curvature. Pupils are forced every day, for several hours, to maintain the same unhealthy posture, overtiring always the same muscles, twisting and bending the spine always at the same place, and thus gradually altering the shape and position of the bones. The normal position would be to keep the upper part of the body straight; the shoulder-blades, both of the same height, freely suspended, together with the upper arm, on the ribs, and in no way supporting the body; both elbows on a level with each other, and almost perpendicular under the shoulder-joint, without any support; only the hands and part of the forearm resting on the table; the weight of the head freely balanced on the vertebral column, and not on any account bent forward, but only turned so much round its horizontal axis that the face may be inclined sufficiently to prevent the angle at which the eye is fixed on the book from being too pointed.

Dr. Liebreich then presents the following design for school desks. The top of the desk has an inclination of 20° for writing; for reading, a greater inclination is required—about 40°. This latter is obtained by turning up a flap of five inches in width, fixed to the front edge of the desk. All seats have backs, consisting only of a board three inches wide, which, placed at the right height, sufficiently supports the lower joints of the spine, to enable the pupil to keep straight while reading or writing. The distance between the back of the seat and the table can be regulated to the size of the pupils, and is always just sufficient for the flap to come quite near the child when writing.


Floating of Solid in Molten Iron.—In a communication to the American Journal of Science, Prof. Adolf Schmidt takes exception to Mr. R. Mallet's explanation of the phenomenon of certain metals in the solid state floating upon a bath of the same metals in a molten state. Mallet assumes the existence of what he terms a "repellent force." Prof. Schmidt upsets this assumption by an experiment which he thus describes: "Have a solid ball of cast-iron of one and a half to two inches diameter cast and filed off pretty smoothly. Have a ladle or vessel of at least three-quarter cubic foot capacity filled with molten cast-iron. If, then, you lay the cold cast-iron ball on the surface of the molten iron, you will find that the ball, in spite of the 'repellent force,' assumed by Mr. Mallet, will sink to the bottom of the ladle at once. With an iron rod you can feel the ball at the bottom of the ladle, and roll it about. But, after twenty or thirty seconds, the ball will slowly rise to the surface of the bath and remain there. It is thus evident that cast-iron, at ordinary temperatures, is both heavier and denser than molten iron, but that, as its temperature rises, the solid iron expands, and becomes lighter, and finally floats on the mol-