170 



SCIENCE. 



[Vol. XIII. No. 317 



appears to me absolutely groundless. The only velocity that can 

 possibly remain constant, or approximately constant, during wheel- 

 ing, is the velocity with reference to the supporting medium ; and 

 as that velocity is, according to his previous assumption, not only 

 small, but negative, there is no energy available to enable the bird 

 to rise. Indeed, the bird, in passing from a negative velocity rela- 

 tive to the air, to a positive velocity relative to the air, must pass 

 through the phase of no velocity relative to the air, in which he is 

 practically helpless, being compelled to fall vertically in order to 

 acquire sufficient speed to steer. Like Mr. Pickering, Professor 

 MacGregor treats the subject as though the earth influenced the 

 motions of a bird on the wing by some other means than gravita- 

 tional attraction. He apparently fails to perceive, that, if the body 

 of air m which the bird moves has no internal motion, its relation 

 to his flight is precisely that of a calm. 



Let me illustrate. A steamer propelled with uniform force on a 

 calm ocean has its rudder turned constantly, and by the same 

 amount, to the right, and consequently describes a circle. This 

 circle is described on the ocean : it expresses a relation between the 

 moving body and that by which it is supported. It has no refer- 

 ence to the bottom of the ocean. It makes no difference whether 

 that part of the ocean is at rest or is part of a swift current. The 

 relation of the boat to the water is not affected by the relative mo- 

 tion of water and bottom. Or consider a skater. Having acquired 

 momentum, he is able to describe circles without propulsive effort 

 until the stored-up energy is consumed by friction and by the re- 

 sistance of the air. The ice on which he circles may be frozen to 

 the shores of a pond, or it may float with uniform speed on a rapid 

 river ; but his relation to the ice is the same in either case, and his 

 circles have the same pattern as engraved on ike ice. The case 

 of the soaring bird is closely analogous. His horizontal motions 

 are related only to the air in which he moves, and by which he is 

 supported, and they are not affected by the uniform horizontal mo- 

 tion of that air with reference to the ground. 



A slight correction, and I have done. I assume, as Professor 

 MacGregor says, that after wheeling, the bird's velocity relative to 

 the medium in which he turns will be the same as before (discount- 

 ing friction); but I do not admit the implication "that during the 

 turn his velocity relative to the earth will change by an amount 

 equal to twice the velocity, relative to the earth, of the medium in 

 which the turn is made." His velocity relative to the earth will 

 change by an amount equal to twice his velocity relative to the 

 medium. G. K. Gilbert. 



Washington, D.C., Feb. 25. 



In two communications published in the last number of Science 

 (p. 151) under the above title, Professor Pickering and Professor 

 MacGregor have developed with considerable ingenuity a theory 

 of the possibility of a bird soaring in a uniform horizontal wind ; 

 but it is certainly true that a bird cannot soar — that is, perma- 

 nently sustain or elevate itself without expending energy — in such 

 a wind, and it has therefore seemed to me to be important, in the 

 interests of clear thinking, to show on dynamical grounds why 

 soaring is impossible in this case. 



Evidently the velocity of the wind relative to the earth has 

 nothing to do with the question, as it is the relative movement of 

 wind and bird that causes the re-actions between them, and there- 

 fore can alone come into consideration. Let the air, therefore, be 

 supposed to be at rest relative to the earth, and it becomes at once 

 obvious that the bird cannot soar : for, suppose the bird to have 

 any imaginable initial velocity, and to wheel in the most artful 

 manner, it is still a mass falling under the influence of gravity, and 

 only resisted more or less by the fluid friction of the medium in 

 which it is placed. This fluid friction of the air against its wings 

 can only delay its fall, but can never prevent it, just as it delays the 

 fall of a feather. 



A theory of soaring must explain how energy is given to the bird 

 by the wind ; but it is clear, that, instead of the bird receiving 

 energy, it is expending either its kinetic energy, as when in one of 

 its whirls it sweeps upwards, or potential energy when it sweeps 

 downwards. But the temporary increase of potential energy in a 

 rise can never equal the corresponding loss of kinetic energy, be- 

 cause energy is being continually expended in frictional heating. 

 There is thus a steady expenditure of energy, and none received 



from the medium, and the bird is therefore bound to come to the 

 ground. The only efifect of the medium is to resist the motion, in 

 whatever direction it may take place, whether up or down. 



As soon as it is clearly seen that the only thing we are concerned 

 with is the relative motion of air and bird, and that the air may be 

 at rest relative to the earth without affecting the question in the 

 slightest, the futility of any attempts to explain soaring in a uniform 

 horizontal wind is apparent. 



If any one wishes to discover the particular fallacies in the theories 

 above mentioned, let him attempt to follow out the reasoning as 

 given in the communications referred to ; assuming, however, that 

 there is no wind, that the air is at rest relative to the earth ; re- 

 membering that the mere fact of the earth's moving relative to the 

 wind has no connection with the relation between the bird and 

 the air. 



The theory that soaring can be kept up by taking advantage of 

 differentially moving layers of air is not open to the above criticism, 

 and may be the true explanation : it is certainly not unreasonable 

 on its face. ARTHUR L. Kimball. 



Johns Hopkins University, Baltimore, Feb. 23. 



To keep Water-Mounts Moist. 



In biological work with the microscope it is frequently desirable 

 to preserve water-mounts for several days, that growth, develop- 

 ment, etc., may be observed from time to time. Water lost by 

 evaporation can be very successfully replaced to the glass slips 

 from a beaker beneath by means of capillary tubes. To make 

 these, hard-glass tubing of about three millimetres bore is softened 

 in a Bunsen flame, and then drawn out to a diameter of from two- 

 tenths to three-tenths of a millimetre. This is then divided up 

 into lengths of five centimetres, and each piece bent at an angle of 

 about 80° one centimetre from an end by holding it over a very 

 small flame for an instant, when, of its own weight, the end falls 

 to the proper angle. One tube is sufficient for a slip, and is applied 

 by first touching the longer limb in water, when instantly the 

 liquid will rise and fill the tube, which may now be suspended by 

 the shorter portion from the glass slip, allowing one end to just 

 touch the edge of the cover-glass, and the other to dip beneath the 

 surface of the water in the beaker. A thin film of water will run 

 along the shorter limb, and hold it securely in place. The whole is 

 then covered with a suitable bell-jar. In this way mountings in 

 water or nourishing solutions may be kept an indefinite time, and 

 are always ready for examination without disturbing them in the 

 least. Should it be desired to supply more fresh water or nourish- 

 ing solution to the mount than would ordinarily arise, a bit of filter- 

 paper applied to the side of the cover-glass opposite the capillary 

 tube will accomplish this. E. B. Knerr. 



Parsons College, Fairfield, lo., Feb. 18. 



Color-Blindness a Product of Civilization. 



The following is a summary of a paper read before the Kansas 

 Academy of Science at Leavenworth, Nov. i, 18SS : — 



The fact that blindness to certain colors exists among civilized 

 people is well established ; also the percentage of cases to be found 

 among males has been determined with considerable probability 

 for the races of Europe and America. There has been much di- 

 versity in methods of testing, and the results of many reported de- 

 terminations might well be called into question. Still it is prob- 

 ably not far from the truth that about four out of every hundred 

 males are more or less deficient in color-sense. Of females there 

 have been reported (B. J. Jeffries, M.D., Color-Blindness, p. 74) 

 as examined in Europe and America 39,828 ; and of these, only 60 

 were color-blind, or 2 per cent. Of both males and females, 

 156,732 have been tested ; and of these, 6,721, or 4.27 per cent, are 

 color-blind. These statistical facts have naturally excited interest 

 and discussion. If so large a number as four out of every hundred 

 are unable to distinguish colors, there arises, of course, a practical 

 question important to the railroads, marine, etc. 



The gravity of this fact is already recognized more or less in all 

 countries by the test examinations for color-blindness among 

 employees. But there is in these statistics also much of interest to 

 scientists. 



