75 



SCIENTIFIC SIDE-LIGHTS 



Bird 

 Birth 





of snails or the flutter of insects is the ar- 

 rival of the migratory birds. That they are 

 migratory is to most of us a matter of such 

 familiar knowledge that we no more think 

 of questioning it than we conceive it neces- 

 sary to doubt the rotundity of the earth or 

 the waning of the moon. We know that 

 certain feathered friends are here during the 

 summer, and it is equally certain that they 

 are absent a few months later, only to ap- 

 pear with the first flowers and the pioneer 

 bees. . . . All summer these little 

 feathered folk revel in the joy of existence. 

 The pair build their nest, rear their young, 

 and disappear, until the observer who was 

 intent six months before in watching their 

 arrival may find a sadder but not less intel- 

 lectual amusement in noting how one by one 

 they vanish from the woods, the commons, 

 the fields, the gardens, and the riversides, 

 where they had to all appearance established 

 themselves for good. BROWN Nature- 

 Studies, p. 11. (Hum., 1888.) 



372. BIRDS' WINGS SELF-ACTING 



VALVES Adjustment to Upward and Down- 

 ward Strokes. But there is another diffi- 

 culty to be overcome [in flight] a difficulty 

 opposed by natural laws, and which can 

 only be met by another adjustment, if pos- 

 sible more ingenious and beautiful than the 

 rest. It is obvious that if a bird is to sup- 

 port itself by the downward blow of its 

 wings upon the air, it must at the end of 

 each downward stroke lift the wing up- 

 wards again, so as to be ready for the next. 

 But each upward stroke is in danger of 

 neutralizing the effect of the downward 

 stroke. It must be made with equal veloci- 

 ty, and if it required equal force it must 

 produce equal resistance an equal rebound 

 from the elasticity of the air. If this diffi- 

 culty were not evaded somehow, flight would 

 be impossible. But it is evaded by two me- 

 chanical contrivances, which, as it were, 

 triumph over the laws of aerial resistance 

 by conforming to them. One of these con- 

 trivances is, that the upper surface of the 

 wing is made convex, whilst the under sur- 

 face is concave. The enormous difference 

 which this makes in atmospheric resistance 

 is familiarly known to us by the difference 

 between the effect of the wind on an um- 

 brella which is exposed to it on the under 

 or the upper side. The air which is struck 

 by a concave or hollow surface is gathered 

 up, and prevented from escaping; whereas 

 the air struck by a convex or bulging surface 

 escapes readily on all sides, and compara- 

 tively little pressure or resistance is pro- 

 duced. And so, from the convexity of the 

 upper surface of a bird's wing, the upward 

 stroke may be made with comparatively 

 trifling injury to the force gained in the 

 downward blow. 



But this is only half of the provision made 

 against a consequence which would be so 

 fatal to the end in view. The other half 



consists in this that the feathers of a 

 bird's wing are made to underlap each 

 other, so that in the downward stroke the 

 pressure of the air closes them upwards 

 against each other, and converts the whole 

 series of them into one connected mem- 

 brane, through which there is no escape; 

 whilst in the upward stroke the same pres- 

 sure has precisely the reverse effect it 

 opens the feathers, separates them from 

 each other, and converts each pair of feath- 

 ers into a self-acting valve, through which 

 the air rushes at every point. ARGYLL 

 Reign of Law, ch. 3, p. 81. (Burt.) 



373. BIRTH OF GEOLOGY Early Study 

 of the Neptunian or Stratified Rocks. In 

 the latter part of the eighteenth century, 

 extensive mining operations in Saxony gave 

 rise to an elaborate investigation of the soil 

 for practical purposes. It was found that 

 the rocks consisted of a succession of ma- 

 terials following each other in regular se- 

 quence, some of which were utterly worth- 

 less for industrial purposes, while others 

 were exceedingly valuable. . . . But 

 while the workmen wrought at these suc- 

 cessive layers of rock to see what they 

 would yield for practical purposes, a man 

 [Werner] was watching their operations 

 who considered the crust of the earth from 

 quite another point of view. . . . From 

 the general character of these rocks, as well 

 as the number of marine shells contained in 

 them, he convinced himself that the whole 

 series, including the coal, . . . the red 

 sandstone, and the Muschel-Kalk, had been 

 deposited under the agency of water, and 

 were the work of the ocean. AGASSIZ Geo- 

 logical Sketches, ser. i, ch. 4, p. 113. (H. 

 M. & Co., 1896.) 



374. 



Button Studies the 



Plutonic or Igneous Rocks. But, in the 

 meantime [compare 373], James Hutton, a 

 Scotch geologist, was looking at phenomena 

 of a like character from a very different 

 point of view. In the neighborhood of Edin- 

 burgh, where he lived, was an extensive re- 

 gion of trap-rock that is, of igneous rock, 

 which had forced itself through the strati- 

 fied deposits, sometimes spreading in a con- 

 tinuous sheet over large tracts, or splitting 

 them open and filling all the interstices and 

 cracks so formed. Thus he saw igneous 

 rocks not only covering or underlying 

 stratified deposits, but penetrating deep 

 into their structure, forming dikes at right 

 angles with them, and presenting, in short, 

 all the phenomena belonging to volcanic 

 rocks in contact with stratified materials. 

 He again pushed his theory too far, and, in- 

 ferring from the phenomena immediately 

 about him that heat had been the chief 

 agent in the formation of the earth's crust, 

 he was inclined to believe that the stratified 

 materials also were in part at least due to 

 this cause. AGASSIZ Geological Sketches, 

 ser. i, ch. 4, p. 115. (H. M. & Co., 1896.) 



