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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 sup- 

 ply an incentive to action by searching for 

 " some inherited seed of ambition or enter- 

 prise 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 simi- 

 lar case, where no such awakening of energy 

 occurred, ended in suicide. 



Tlie Mirage on Swiss Lakes. Profes- 

 sor 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 

 thnn 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 experi- 

 mentally. 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 illu- 

 sions. 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 Ville- 

 neuve, the steamboat plying between Mon- 

 treux and Yevay 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 re- 

 fracted 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 irregu- 



larly. Then the rays of light undergo ab- 

 normal refractions which are not always the 

 same for the upper and lower parts of ob- 

 jects. Consequently, the objects are some- 

 times 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 there- 

 by 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 reflec- 

 tion 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 tlie Fixed Points of Ther- 

 mometers. M, Crafts, in the course of his 

 investigation of the causes of the variations 

 of the fixed points of thermometers, has dis- 

 covered 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 tempera- 

 ture, retains a part of the expansion which 

 it has undergone. By heating it anew to an 

 inferior temperature, say 570, we may pro- 

 duce a new diminution of volume, and thus 

 successively, by a very slow process of cool- 

 ing, 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, ac- 

 cording 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 ex- 

 ample, which gives a depression of half a 

 degree after a long exposure at 212', ought, 

 by this law, to give at G70 a depression of 

 6.8. The depressions actually observed 

 are much less considerable. 



