246 



SCIENCE. 



[Vol. XIII. No. 321 



Suppose, now, that he is drifting towards B with a velocity equal 

 to that of the wind, and travelling at right angles to AB with 

 such a velocity that he can move along horizontally without fall- 

 ing towards the earth. Suddenly a gust overtakes him from 

 the direction of A. He at once turns towards it, and his velocity 

 relative to it is sufficient to raise him in the air. It tends to 

 carry him more rapidly towards B ; and when his velocity relative 

 to it has sunk to the same value as before, and he again travels 

 horizontally, he turns again at right angles to the line AB, but in 

 the opposite direction to that which he had before. Presently the 

 force of the gust diminishes, and the wind seems to blow towards 

 him from the direction A. He accordingly turns towards it again, 

 rising from the ground till his velocity relative to the air has as- 

 sumed its former value, and he moves horizontally, turning again at 

 right angles to the line AB, and the cycle is completed. He thus 

 moves along in the direction AB with a mean velocity equal to that 

 of the wind, rising when moving parallel to it, and moving horizon- 

 tally, or perhaps slowly falling, if the gusts do not come with suffi- 

 cient frequency, when moving at right angles to it. 



In the case of all soaring birds, the spread tail, being an inclined 

 curved surface, presents a large area to the wind. As it is situated 

 at a considerable distance from the bird's centre of gravity, it must 

 convert him into a sort of floating weather-cock, the wings serving 

 as dampers to restrain him from turning too quickly. It therefore 

 appears, if soaring really does depend on the interaction of varying 

 wind-currents, as if the changes of direction involved must be 

 almost automatic, and not a thing which the bird is required to 

 learn ; although he may doubtless learn to take advantage of favor- 

 ing currents by giving proper inclinations to his wings and tail. 



If the question be raised as to the sufficiency of the varying in- 

 tensity of the wind-currents to maintain the bird's initial velocity 

 against the resistance of the air, we must reply that it is a matter 

 which can only be determined conclusively by e.xperiment. Certain it 

 is, however, that in windy weather the wind does come in gusts. If 

 in the course of his circles the bird happens to be travelling at right 

 angles to the wind, when the gust strikes him he will surely be turned 

 round, almost in spite of himself, so as to face the gust. If the 

 bird does face the gust, it will certainly raise him to a higher level. 



If this explanation proves to be the true one, the reason why 

 small birds cannot soar is probably, that, in those of them that have 

 suitably shaped wings and bodies, their surfaces are so large in 

 proportion to their weights that they rapidly assume the velocity of 

 the surrounding air. In order that they might soar to advantage, 

 the gusts should come more frequently, and be of shorter duration, 

 than we actually find to occur in nature. 



Wm. H. Pickering. 



Harvard Obser\'atory, Cambridge, Mass., March 21. 



the method is valueless. All compound words and phrases con- 

 nected by hyphens were counted as single words only. The 300 

 sentences filled 30 out of 200 pages of the edition used. 



H. A. Parker. 



Cambridge, Mass., March 25. 



Definition of Manual Training. 



I HAVE just seen in your pages {Science, xiii. p. 9) the excellent 

 definition of " manual training," given by the New Jersey Council 

 of Education. But the name is already too familiar in various 

 vaguer uses, and especially for training to fit for manual labor: 

 hence there would be great advantage if afresh name were applied. 

 Would not " manu-mental training " do admirably? It expresses 

 the precise idea in such a way that a mistake as to its meaning is 

 impossible. J. E. Clark. 



Bootham, Yorli, Eng., March 15. 



Curves of Literary Style. 



After reading the communication on " Curves of Literary 

 Style," in the last number of Science, I counted the words in 300 

 sentences towards the last of Carlyle's " French Revolution," and 

 found the curve, when plotted, to agree very closely with your cor- 

 respondent's as published, though there were several longer sen- 

 tences interspersed, showing that the passages examined were from 

 a different part of the work. This was very satisfactory ; but the 

 same method of examination, applied to the first 300 sentences of Car- 

 lyle's " Sartor Resartus," gives a very different result, the curve cor- 

 responding pretty closely with that given for Johnson's " Rambler." 

 This goes to show, if it does not prove, that for detective purposes 



The Velocity of Storms as related to the Velocity of the 

 General Atmospheric Movements. 



It has for a long time been maintained by some meteorologists 

 that the chief cause of the progressive movement of storms is that 

 these atmospheric disturbances are carried along by the general 

 movements of the atmosphere, as eddies on the surface of a river 

 are borne along by the current in which they exist. The German 

 meteorologists Van Bebber and Koppen have especially insisted on 

 these views, maintaining that the direction and velocity of storms 

 are determined by the mean motion of the entire atmosphere in 

 which they exist ; and Gen. Greely has recently, in the American 

 Meteorological Journal, educed the recorded wind-velocities on 

 Mount Washington as favoring this view. 



In order to study this and allied questions, the writer began two 

 years ago a systematic series of observations on the clouds. These 

 observations were made hourly between 7 A.M. and 11 P.M. Facil- 



Wind Velocity 



Mount Washington 



6,279 Feet. 



Wind Velocity 

 Blue Hill 

 655 Feet. 



Westerly Velocity 

 Blue Hill 

 65s Feet. 



ities were not available for obtaining the actual velocities of the 

 clouds, and it was hence necessary to be content with obtaining 

 the apparent velocities. These were obtained by means of a 

 nephoscope devised by the writer. The nephoscope consists of a 

 horizontal mirror held in a frame carrying an eye-piece movable 

 along vertical and horizontal arches, so that the direction of cloud- 

 movements can be determined in degrees of azimuth. To obtain 

 the relative velocity, a movable support is so arranged, that, when 

 the observer's forehead is rested on it, the retina of the eye is 

 maintained at a constant height of seven inches above the surface 

 of the mirror. When the eye is in this position, the number of 

 quarter-inches which the image of a cloud is seen to move across 

 the mirror in a minute is taken as the relative velocity of the cloud. 

 It is evident that the relative velocity of the cloud thus obtained 

 bears a relation to the actual velocity ; and, if the height of the 

 cloud be known, its absolute velocity relative to the earth's surface 



