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THE POPULAR SCIENCE MONTHLY. 



the great eccentricity of the orbit of Mars, 

 and the consequent heavy fall it makes when 

 plunging toward the sun. Situated farther 

 from the sun than we are, Mars must be re- 

 garded as an older member of our system ; 

 and since it is smaller than the earth, it is 

 only natural that its surface crust should be 

 thicker than that of our planet. Granting 

 this, then the internal pulp would not have 

 such a power to compensate for the rapid fall 

 as the earth does internally, for there would 

 not be so much of it, so that an external com- 

 pensation, assuming the crust to be too thick 

 to alter its form, would have to take place at 

 the surface. On the surface, of course, the 

 water is the only power ; therefore we should 

 expect, to put it in Mr. Kingsmill's own 

 words, " that the water in the ocean would 

 be projected into the Martial hemispheres, 

 and as the planet approached the sun, tides 

 would sweep round the planet ; that the ca- 

 nals should sometimes appear and sometimes 

 be duplicated ... is only, a priori, what 

 might be anticipated." 



Factors of a River's Character. — Where 

 a river shall go, what kind of a channel it 

 will cut, how much work it will do, says A. 

 P. Brigham, in his paper on Rivers and the 

 Evolution of Geographic Forms, are matters 

 determined, in an infinite number of ways, 

 by the underlying strata. A river flowing on 

 horizontally bedded rocks will tend to have, 

 in its youth, a narrow canon. Alternations 

 of hard and soft strata give, in early stages 

 of river life, alternations of rock benches and 

 talus slopes ; and many terrace-like horizons 

 on the sides of the valley mantled commonly 

 by soil, have this origin. Thus a terrace may 

 be built up or carved out, and it may consist 

 of alluvium, glacial rubbish, or bed rock. 

 Tilted rocks give different types of river val- 

 leys in infinite variety. These types may be 

 said to be just now beginning to attract a fair 

 share of the interest of geographers and ge- 

 ologists. They will, in years to come, afford 

 some of the most intricate as well as most 

 fascinating problems which are open to in- 

 quiry. 



The Critical Point in a Thunderstorm. — 



The belief that danger from lightning ceases 

 as soon as the rain begins to fall heavily — 

 expressed in the words of a mother reassur- 



ing her children, " Don't cry any more, God 

 is sprinkling the earth with holy water " — 

 prevails extensively among the Flemish peas- 

 ants. Usually, according to M. P. J. De Rid- 

 der, lightning flashes from storm-clouds at 

 the line between the heavy rain of large 

 drops and the finer rain — or from the edge 

 of the heavy rain. This is always the case 

 in cumulo-nimbus storms, and as the number 

 of storms of that kind exceeds all others, the 

 belief of the peasants is at least worthy of 

 attention. In nimbus storms, on the other 

 hand, the critical point is at the latter end. 

 In those of them which are developed in the 

 veil of strato-cirrus, as when the sky is slow- 

 ly covered, the rain falls at first without in- 

 tensity and increases gradually, with distant 

 thunder, while the storm itself does not seem 

 to make much headway. But suddenly the 

 rain falls more rapidly, and the dangerous 

 moment has come. The roar of the thunder 

 becomes terrible, the storm ceases, and the 

 sky is cleared. 



The Agaves. — The name of aloes is com- 

 monly given to plants of peculiar appearance 

 which have long, fleshy leaves, with spines 

 on their tips and sides ; but this does not ex- 

 plain why the name has been given to species 

 to which it does not belong, such as the agave, 

 which do not resemble them. In Central Amer- 

 ica, their real country, where they have been 

 cultivated on a large scale from the most re- 

 mote times, they are called pitu, ozal, istle, 

 metl, maguey, etc. Probably, soon after the 

 discovery of America, a species of agave was 

 introduced in the south of Europe, and be- 

 came quite at home there. Linnaeus, not 

 realizing that all the agaves are American, 

 gave it the specific name of Agave ameri- 

 cana. Now there are more than a hundred 

 species on horticulturists' catalogues, but 

 many of these are only varieties. The uses 

 to which these plants are found applicable 

 are constantly increasing. In the United 

 States and Europe they are only garden orna- 

 ments. In Mexico they hold the first place 

 as wine plants and as textile plants. The 

 filamentous substance obtained from their 

 leaves is known all over the world as aloes 

 fibers. These fibers, of length and thickness 

 depending on the variety and locality, are so 

 elastic and durable as to be in great demand 

 for ropes, brushes, harness, and coarse woven 



