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THE POPULAR SCIENCE MONTHLY.

tune bringing pressing need for exertion; but this remedy is beyond the reach of a physician. He might aim, however, to supply an incentive to action by searching for "some inherited seed of ambition or enterprise which has never yet germinated," and may sometimes find it by learning the story of the father's or grandfather's life. A case which came under Dr. Granville's care, and which furnished him with the basis for his remarks on this subject, was cured by the awakening of a strong passion for the breeding of stock, which he had inherited from his grandfather, but which had not been aroused in his nature till he was thrown into circumstances which excited him to emulate the success of a neighbor. A similar case, where no such awakening of energy occurred, ended in suicide.

 

The Mirage on Swiss Lakes.—Professor Charles Dufour communicated to the French Association, at its last meeting, a paper on the mirages of the Swiss lakes, which are often seen between the month of August and the spring, especially in the morning, when the water is warmer than the air. When Monge published his explanation of the mirage, he supposed that the strata of air near the ground were warmer and rarer than the strata above, but he could not prove it experimentally. Professor Dufour has proved it by taking the temperature at different heights above Lake Leman, while the sun was still hidden by the mountains. The mirage frequently produces curious illusions. When a boat is near the point where the ray of light is a tangent to the surface of the water, the mirage of the sky is thrown below the boat, and the latter seems to sail in the air. Seen from Villeneuve, the steamboat plying between Montreux and Vevay seems to be sailing among the vineyards which cover the hills along the shore. When the air, on the other hand, is warmer than the water, as is the case generally in the spring and summer on fine afternoons, the concave side of the refracted ray of light is turned toward the water, and objects are brought into sight which are really hidden by the roundness of the earth. Sometimes the temperature of the different strata of the air varies irregularly. Then the rays of light undergo abnormal refractions which are not always the same for the upper and lower parts of objects. Consequently, the objects are sometimes diminished, sometimes magnified in an extraordinary fashion. Small houses thus distorted are made to look like palaces; their white color is changed into gray by the diffusion of the light, and they are thereby given an air of greater grandeur. Many persons fail to take notice of these mirages because they regard them as reflections from the water; but it is really possible for one with his eye near the water to see the reflection from it of a distant object on nearly the same level. When an image of such an object is seen, it is most probably a mirage.

 

Variations in the Fixed Points of Thermometers.—M, Crafts, in the course of his investigation of the causes of the variations of the fixed points of thermometers, has discovered that glass heated for a long time in the blowpipe-flame shrinks in consequence of an internal change. It is not shown that pressure plays any part in the phenomenon. The particles of the glass which have been separated by the heating do not return to the normal position immediately on cooling, but appear to be in a disturbed condition for some time afterward. The action of heat at a given temperature, say of 670°, by giving a greater mobility to the particles, favors their return to the normal position; but the glass, in cooling from this temperature, retains a part of the expansion which it has undergone. By heating it anew to an inferior temperature, say 570°, we may produce a new diminution of volume, and thus successively, by a very slow process of cooling, bring about the greatest approximation to the normal state, and consequently the greatest stability. The law discovered by M. Pernet for temperatures between the freezing and boiling points of water, according to which the depressions of the freezing-point are proportional to the squares of the temperatures, is not true at high temperatures. A thermometer, for example, which gives a depression of half a degree after a long exposure at 212°, ought, by this law, to give at 670° a depression of 6.8°. The depressions actually observed are much less considerable.