March 15, 1889.] 



SCIENCE. 



205 



correct for A 21. The use of the factor 3 for all patterns of ane- 

 mometers now for nearly thirty years has introduced a great 

 amount of error in published wind velocities ; so that they are not 

 only not comparable generally with one another, but the errors 

 have likewise affected most, if not all, the results obtained from the 

 discussions of these velocities. It is much to be regretted, there- 

 fore, that some standard pattern had not been adopted and its con- 

 stant accurately determined at the start, instead of deferring it for 

 nearly thirty years ; for, if this is even now done, it will be a long 

 time before any adopted standard and its true constant can come 

 into general use. 



Since the force of the wind is as the square of the velocity, errors 

 in the estimated velocity of the wind give rise to errors in the 

 pressure of the wind which are proportionately more than twice as 

 great. For instance : if the true velocity of the wind is 30 miles per 

 hour, the Kew Standard with its factor 3 makes it 42 miles nearly, 

 an increase in the ratio of i to 1.4; but the force of the wind is 

 increased in the ratio of 30" to 42-, or as i to 2 nearly, and so in a 

 ratio more than double the preceding one. In estimating the force 

 of the wind from the indications of the anemometer, the effect of 

 the error in the factor 3 of the anemometer, and of the wind-press- 

 ure constant .005, now in general use, are both in the same direc- 

 tion ; so that the combined errors of both are very great. For in- 

 stance : in the case of a wind of 30 miles per hour, we have seen 

 above, that the error of the factor 3 applied to the Kew Standard in- 

 creases the force of the wind in the ratio of i to 2 ; and if the wind- 

 pressure constant should be .003 instead of .005, then the effect of 

 both errors is to increase the estimated force of the wind above the 

 true force in the ratio of i to 2 x |, or to more than three,times the 

 real force. Of course, this is an extreme, but not an impossible 

 case ; for in anemometers mostly used the error of the factor 3 is 

 not nearly so great as for the Kew Standard, and the true value of 

 the wind-pressure constant may come out a little more than .003 

 when accurately determined, but still the errors of estimated wind 

 forces, with the constants in use, are undoubtedly enormously 

 large. Mr. Whipple, of the wind-force committee, says, that, 

 " unless the Robinson anemometers could be put into the hands of 

 those who would take care of them, their indications were fre- 

 quently worse than useless. The instruments require to be con- 

 tinually looked after. Even if carefully attended to and regularly 

 cleaned and well oiled, their records are far from satisfactory." 



It is the opinion of the writer that they must in time give way to 

 something better, probably to Mr. Dines' newly invented helicoid 

 anemometer, which is more simple in its mechanical action, and, 

 according to the experiments made with it, seems quite satisfac- 

 tory. A description of this instrument is found in the Quarterly 

 Journal of the Royal Meteorological Society for July, 1887. 



Wm. Ferrel. 



The Soarings of Birds. 



There can be no doubt that the explanation of soaring given by 

 Mr. Gilbert is mechanically sound. The only remaining question 

 seems to be as to its sufficiency. In regard to this question, the 

 following considerations may be of service : — 



There is a certain velocity relative to the air such that a bird 

 possessing it can be sustained against gravity without muscular 

 exertion. Let F represent that velocity for a given bird. Let there 

 be two horizontal layers of air, whose relative velocity is i. For 

 simplicity, let the velocity of the lower layer be zero, that of the 

 upper i. Suppose the bird at some instant to be in the upper layer, 

 moving in the same direction with it, and with a velocity relative to 

 it of V, so that he can just be sustained while moving horizontally. 

 His velocity relative to the lower layer is V+i. Let him now 

 descend into the lower layer and wheel horizontally 180 degrees. 

 In so doing he necessarily loses some energy, and his velocity de- 

 creases. Now, in order that he may be sustained at the same level 

 during the wheeling, his velocity relative to the lower layer must 

 not fall below V. Suppose his decrease of velocity to be a little 

 less than i : he will then be moving opposite to the direction of 

 the upper current, with a velocity greater than V. He can there- 

 fore not only maintain his level, but can rise. Let him now enter 

 the upper layer, his velocity relative to it being V+i. If, now, he 



can wheel horizontally through 180 degrees without losing more 

 than the velocity i. he will be in a position to repeat the cycle. 



The statement of Professor Oliver in Science (xiii. p. 16) seems, 

 to imply^that the difference in velocity of the air-currents needs to be 

 as great as the relative velocity which will enable the bird to sustain 

 himself against gravity ; that is, that i must be as great as V. If 

 the discussion here given is correct, such is not the case. It is- 

 only necessary that the bird should have initially a sufficient rela- 

 tive velocity, and should be able to wheel horizontally 180 degrees, 

 without losing by " friction " enough energy to reduce his velocity 

 as much as i, the velocity of one air-current relative to the other. 



L. M. HOSKINS. 

 University of Wisconsin, Madison, March 5. 



" Shall We Teach Geology ? " 



When a reviewer bases critical verdicts on ignorance or misap- 

 prehension of the work reviewed, he has an advantage over the 

 author, of which, in my own experience, I usually leave him in 

 quiet possession. Still the meekness of silence may not always 

 prove most useful to the public. Your reviewer of my work, "Shall 

 We Teach Geology.'" in No. 317, says that I ignore the mental 

 and moral sciences as means of culture ; but he should have ob- 

 served that I do not undertake to discuss the education value of all 

 sciences and literatures, but only of those selected as types by cer- 

 tain pedagogical writers who hold geology in disesteem. Your re- 

 viewer states that I mention " history only to slight it, declaring 

 that it trains no faculty but verbal memory." My criticisms on 

 history contemplate it as a study urged upon children in the early 

 stages of education. This is what I have recorded on purpose to 

 forestall such an accusation. " My present investigation concerns 

 studies as usually taught and in schools of the lower orders. In 

 college, history and literature are pursued in a nobler and more 

 cultural way " (p. 148). Your reviewer employs the term " litera- 

 ture " in the wide sense, which makes it a much more valuable 

 thing than literature as used in the narrow sense of the author^ 

 whosejpositions I am examining (note, p. 145). Your reviewer 

 states, also, that I claim for geology that " the subject should be 

 taken up in the primary schools, and pursued every year as long as 

 the student attends school." This is preposterous criticism. Such 

 is not my position, nor is the idea anywhere conveyed. I think the 

 subject should be taken up briefly, two, three, or more times, at 

 successive stages of mental development, not completed in one 

 course late in school-life (see pp. 133, 134). 



Alexander Winchell. 



Ann Arbor.'Mich., March 5. 



To keep Water-Mounts Moist. 



In my last communication on this subject {Science, xiii. p. 170) I 

 recommended glass capillary tubes. I since find that a much 

 simpler plan, and one that serves equally well in most cases, is to 

 suspend from the edge of the cover-glass, to a beaker of water be- 

 neath, a moistened piece of filter-paper about four centimetres 

 long and half a centimetre wide. 



Likewise, in the study of germination of seeds, the capillary 

 tubes or the moistened filter-paper may be put to good service. 

 Very clean and satisfactory specimens of the first stages of germi- 

 nation may be obtained by placing the moistened seeds in contact 

 one with another on a glass slip over a beaker of water, and sus- 

 pending from their midst to the water one of the tubes or simply a 

 narrow piece of paper. A bell- jar will exclude dust. 



E. B. Knerr. 



Parsons College, Fairfield, lo., March 6. 



The Wind-Pressure Constant. 

 In my note I see you have put Hazen for Hagen. The latter is 

 a German physicist of Berlin. Will you please make the correction 

 in your next number ? This is important, since Hazen has also 

 made experiments, the results of which differ very much from 

 Hagen's, and it may seem that I have misrepresented his results. 



Wm. Ferrel. 



Kansas City, Mo., March 5. 



