September 4, 1890^ 



NATURE 



457 



All dull colours, such as browns, olives, plums, &c., mean 

 that vibrations of every wave-length in the white sunlight 

 are absorbed almost entirely, a very small proportion being 

 reflected. A deep red colour means that there is a less propor- 

 tion of the longest waves absorbed ; a deep violet, that there is 

 a less proportion of the shortest waves absorbed ; and a full 

 green, that the absorption is less in the intermediate wave- 

 lengths. These are the primary hues ; but in objects which 

 reflect the brilliant secondary hues — scarlets, yellows, blues, and 

 ])inks — the chief absorption is confined to a small area in the 

 spectrum, a large proportion of the light being reflected. 



There are, then, three distinct stages of coloration, viz. (l) 

 that in which all wave-lengths are absorbed; {2) that in which 

 .ibsorption ceases in respect to about one-third of the spectrum ; 

 (3) that in which absorption ceases in respect to about two-thirds 

 of the spectrum. 



These three stages are progressive, and in the direction of 

 progress from chaos to unity ; from a condition of the proto- 

 plasm in which molecular elements of very diverse vibrating 

 capacity are mixed up together, to a condition in which the 

 capacities of these elements have become greatly simplified. 



When we speak of an organism arriving at maturity, we imply 

 that it began its career in a state of immaturity, and that it 

 gradually progresses to the condition of maturity. In what that 

 condition consists, or what fundamental changes have taken 

 place, it may not be easy to say ; but it is surely true, as a rule, 

 that organisms in an early and immature state are comparatively 

 dull in colour, and do not put on their brightest hues until the 

 period of maturity, indicating that one of the characteristics of 

 maturity is the simplification of the vibrating capacity of the 

 molecules. If this be really a law of Nature, it is a far- 

 reaching one, and will account for much. F. T. MoTT. 



Leicester. 



On the Soaring of Birds. 



I HAVE thought that this habit can be explained as follows ; 

 at least as regards rooks, which I have often noticed soaring in 

 flocks, especially in the spring, and I think usually in warm 

 cloudy weather. 



An upward convection current of warm air is established over 

 some area. The birds stretch out their wings, and if the upward 

 velocity of the current should happen to be just equal to the velocity 

 with which a bird with outstretched wings would sink through still 

 air (the "terminal velocity"), the bird would be supported ; but if 

 it were somewhat greater, the bird would be raised upwards. In 

 that case he inclines his wings so that the resolved part of the 

 pressure on the under side of the wings carries him forward at a 

 uniform level. But this movement, being rectilinear, would take 

 him outside the warm column which he is enjoying. A centri- 

 petal force is therefore needed to maintain the circular move- 

 ment, and this is obtained by tipping the wings, so that the wing 

 which points outwards is raised, and that which points inwards 

 towards the warm column is depressed, as noticed by your cor- 

 respondent. If the upward velocity of the current is not sufficient 

 to support the bird, an occasional flap with both wings, and the 

 subsequent sinking, supplies the deficiency of upward pressure. 



O. Fisher. 



In your i-sue of August 21 (p. 397) Mr. Magnus Blix gives a 

 very ingenious explanation of the soaring of birds. It appears, 

 however, to me that this explanation rests upon a false basis. 



In his illustration, Mr. Blix supposes a bird to be moving in a 

 direction, relative to the wind, at right angles to that of the 

 wind, its absolute velocity, therefore, being greater than that of 

 the wind. He then supposes the bird, by movement of wing- 

 jjlane, to change its direction to one opposite to that of the 

 wind, and assumes that its absolute velocity, in the new direction, 

 will be equal to the absolute velocity in the old. 



Now it is probably true that a bird can change its direction 

 without sensible loss of velocity relative to the air, but any 

 velocity it may have, in virtue of the motion of the air, must 

 remain as a component of the new velocity in the same direction 

 as before, however the bird may change the direction of its 

 wing-plane. 



Now the supposed bird, in changing its direction at c, would 

 still have the component of velocity due to the wind acting in 

 direction ef as before. Its velocity relative to the wind, there- 

 fore, from c io d would be the original velocity at a (diminished 



NO. 1088, VOL. 42] 



in its passage from a Xo c) ; its absolute velocity the diflference of 

 the two velocities. 



If this objection hold good, Mr. Blix's theory seems to be 

 no longer an explanation. C. O. Bartrum. 



19 Well Walk, Hampstead, August 26. 



Occurrence of a Crocodile on Cocos Islands. 



During a recent visit to Cocos Islands Mr. Ross showed me 

 the skull of a crocodile of small size which had appeared about 

 a year previously on the islands. It was first seen by a native 

 Cocosian, who reported that he had seen something between a 

 lizard and a log of wood in the sea. It then reappeared upon 

 another island and destroyed a number of ducks, and was 

 eventually shot by Mr. Ross. The distance from Java, the 

 nearest land, is fully 700 miles. It is remarkable that this 

 animal should have swum so far, and managed eventually to 

 strike this small patch of land in the middle of the ocean. I do 

 not know another record of a big reptile travelling so far. Mr. 

 Ross tells me that bamboo-rafts sometimes drift to Cocos, and 

 perhaps it managed to help itself along on one of these. 



The whole seas here, but especially the Straits of Sunda and 

 Malacca Straits, are full of drift-fruits, seeds, sticks, stems of 

 Nipa and Pandanus ; and between the Straits of Sunda and 

 Cocos, large patches of pumice rolled lumps and dust can be 

 seen, the relics of the destruction of Krakatab. 



H. N. Ridley. 



Botanic Gardens, Singapore, August 6. 



, Helix nemoralis and hortensis. 



I SHOULD be very pleased if some of the various concho- 

 logical readers of Nature would kindly furnish me with 

 their records of these two shells. The questions I specially 

 want to ask concerning them are as follows : — What varieties 

 (with band-formulas) have they found ? What number of each 

 variety and band- variation have they taken ? What is the environ- 

 mental condition of the localities where they have found them, 

 as regards plant-life and geological formation ? And, in addi- 

 tion, I want the records (and this is a special point) from 

 separate and distinct hedges or banks. 



J. W. Williams. 



57 Corinne Road, Tufnell Park, N. 



Mr. Williams's "British Fossils." 

 In my review of Mr. Williams's " British Fossils," published 

 in Nature of August 28 (p. 412), I notice a slip on my part 

 in regard to eclogite. I should have said that whereas this rock 

 is stated to consist of red garnets and hornblende, it is usually 

 described as being composed of red garnets and one of the 

 pyroxenes, such as omphacite or smaragdite, or both. 



Since writing the review I have come to the conclusion that 

 the twice repeated term "dermoid types" is intended for 

 " demoid types " ; a term used in the second edition of Phillips's 

 " Manual of Geology." The Reviewer. 



August 29. 



A Remarkable Rainbow. 



I HAVE just seen a very remarkable rainbow. It was plus 

 60° in height, and thin. The sunset was lurid, with a mock sun 

 to the south of the real one. D. MacGillivray. 



Oxford, August 25. 



NOTES. 

 On Sunday, August 17, M. Janssen ascended to the Grands 

 Mulets, and next day he reached a hut called the Cabane des 

 Bosses, which an Alpinist, M. Vallot, of Paris, has erected at a 

 point about 400 metres below the summit of Mont Blanc. 

 According to the Paris correspondent of the Times, the second 

 day's journey was made in a sledge, drawn and pushed by twenty- 

 two guides. Tuesday, Wednesday, and Thursday M. Janssen 

 spent in a part of the hut which M. Vallot has fitted up as a 

 scientific laboratory. On Friday, as the weather was very clear, 

 M. Janssen had his sledge dragged up to the summit of the 

 mountain to complete his observations. At the ridge of the 



