October 9, rSgi.] 



SCIENCE. 



205 



and diminishing the velocity which would otherwise be 

 found there." 



Then follow the determinations of Boileau and of Bazin, 

 from which it may be inferred " that the ratio at which the 

 maximum velocity is found to the whole depth ranges from 

 zero to 0.2, except in some artificial channels, where it 

 reached 0.35. The Mississippi experiments give different re- 

 sults, and Bazin inclines to believe that the method of ex- 

 'perimenting was untrustworthy. The ratio is greatest in 

 artificial channels with smooth bottoms, and least in natural 

 streams with rough bottoms. 



It is difficult to understand what force could cause the 

 portions of water retarded by the sides or bottom to spread 

 themselves with constant uniformity over the unimpeded 

 current flowing below the surface in mid-stream, and espe- 

 cially how the portions retarded by the bottom could rise 

 up through or pass around the more rapid portions above 

 them. But the phenomenon becomes very simple if we 

 suppose that each molecule of the water has its own proper 

 motion, governed by well-known mechanical laws. The 

 impetus to the motion is determined by the pressure, and the 

 actual motion is necessarily the resultant of the difference 

 between the pressure and the resistance. If there were no 

 resistance to the flow of the stream, there would be constant 

 acceleration of motion from top to bottom, just as there is 

 in jets from the side of a vessel, the flow from each being 

 determined by the pressure above it. But in a flowing 

 stream there is great resistance from the sides and bottom, 

 the resistance from the bottom necessarily increasing with 

 the pressure, and this resistance which the molecules receive 

 from the bottom is transmitted, just as pressure is from above, 

 to the molecules adjacent to them. At the depth where the 

 impetus to motion by the pressure from above comes into 

 equipoise with the resistance to motion from below, there 

 ought to be, as there is in fact, the greatest velocity of flow. 

 The resistance from the bottom r&mains practically constant 

 at any given place in the stream. Wind blowing up stream 

 increases the pressure by holding back the surface mole- 

 cules; hence this increase of pressure, the resistance remain- 

 ing constant, causes the level of maximum velocity to de- 

 scend. On the other hand, when the wind blows down- 

 stream there is a diminution of pressure, because the surface 

 molecules are pushed forwards in the direction of their 

 movement; hence this diminution of pressure, the resistance 

 still remaining constant, causes the level of maximum ve- 

 locity to ascend. When the flow is through a round pipe 

 entirely filled with water, and under such pressure that the 

 influence of gravity on the stream itself may be disregarded, 

 it is obvious that the maximum velocity is through the cen- 

 tre of the pipe; the pressure is uniform in all parts of a 

 cross section of the pipe, and the resistance from friction 

 against the pipe is likewise uniform in all directions from 

 the centre. 



It is not necessary to seek further evidence of molecular 

 motion in other phenomena of hydraulics. The evidence is 

 manifest in all the phenomena that I have examined; and 

 the inotion is not only consistent with the facts, but the hy- 

 pothesis of its existence clears up many things which with- 

 out it are obscure. The explanation which it furnishes of 

 the phenomena of wave motion is especially interesting, but 

 the subject is too large for consideration in this paper. 



It seems to me, therefore, that, without further illustra- 

 tion, we may assume as determined that, in all flowing, the 

 particles or molecules constituting the body in which the 

 phenomenon occurs, whether visible or invisible, have each 



its own proper motion, determined by the forces and resis- 

 tances to which it is subjected, and that the molar motion is 

 made up of the aggregation of these molecular or particle- 

 motions, — and in this consists the specific difference be- 

 tween flowing and sliding. 



This determination is evidently of theoretical importance 

 in hydromechanics and in pneumatics, for the law must ap- 

 ply to the flowing of gas as well as to the flow of liquids, 

 and it may lead to other determinations of great practical 

 value in one or both of these sciences. But since Mr. 

 Crookes has put the molecules of residual gas, in the bulb 

 of the radiometer and in his tubes, to doing mechanical 

 work, the basis has been laid for the development of the 

 Science of molecular mechanics, and it is in this new field 

 that this determination has its greatest importance. 



The eyes of scientists are being directed to what we might 

 call the small end of nature, and we are discovering that 

 microbes, bacilli, bacteria, etc., are of more importance to 

 mankind than the cedars of Lebanon, or the beasts which 

 roamed beneath them, or the birds which sought shelter in 

 their branches. So in this new science of molecular me- 

 chanics, the way to which has been opened up to us by Mr. 

 Crookes's researches, we have the promise of additions to 

 scientific knowledge more important even than the magnifi- 

 cent results which followed the application of mechanical 

 laws to the movements of the celestial bodies. 



Daniel S. Troy. 



LETTERS TO THE EDITOR. 



#** Correspondents are requested to be as brief as possible. The writer^ s name 

 is in all cases required as proof of good faith. 



On request in advance, on"- hundred copies of the number containing his 

 communication unit be furnished free to any correspondent. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



Rain-Making. 



In the issue of Science of Aug. 28 there appears a communica- 

 tion from Professor H. A. Hazen attacking the artificial rain 

 theory, to some points in which I ask the privilege of making re- 

 ply- 



Professor Hazen commences by saying that " ever since the time 

 of Plutarch the idea has been prevalent that great battles are in- 

 variably followed by I'ain." Now, I would ask where Professor 

 Hazen gets his authority for this broad and sweeping statement? 

 In what writings, following those of Plutarch, does he find any 

 reference to the matter up to the time of Benvenuto Cellini, who 

 is said to have written that a discharge of artillery affected mete- 

 orological conditions? Plutarch lived in the first century of the 

 Chi-istian era, Cellini lived in the sixteenth century. Here is a 

 great gap of about fifteen hundred years, and if there is any evi- 

 dence that the idea prevailed, during that time, that battles caused 

 rain, I challenge my critic to produce it. 



A great many writers besides Professor Hazen have brought 

 forward the statement of Plutarch relative to rains followmg bat- 

 tle.-i as an argument against the concussion theory of rain-produc- 

 tion, and some appear to think the argument quite unanswerable. 

 It is, however, very easily disposed of, for the notion referred to 

 by Plutarch was an entirely different matter from that which, so 

 far as we know, did not come into notice anl'il fifteen hundred 

 years later. It was wholly different, in that it did not relate to 

 rains immediately following battles. The only place in which 

 Plutarch mentions the subject is in his life of Marius, in speaking 

 of the defeat of the Ambrones by the Romans. The rains which 

 be says followed that battle did not occur until the winter follow- 

 ing. And in mentioning the subject in a general way in connec- 

 tion with this one specific instance, the whole tenor of what he 

 says conveys the idea that the rains he referred to did not occur 

 until a considerable time after the battles, nor imtil the bodies of 

 the slain had pulritied. To give what he says other meaning is to 

 make his attempted explanation of the cause of the rains whoUy 



