248 



SCIENCE 



[Vol. XVIII. No. 456 



boat on the side. When the life-preservers were put on, I opened 

 my satchel and slipped my travelling flask into my overcoat 

 pocket, leaving money and other valuables to their fate, and took 

 position with my wife, who was one of the party, on the deck (a 

 portion of which in rear of the little cabin was sheltered), so as 

 not to be carried down by the boat if it capsized, and held on to a 

 post to prevent being blown overboard. After these precautions 

 were taken, I looked out on the lake, and to my unbounded as- 

 tonishment the surface was almost perfectly smooth. The mo- 

 ment an incipient wave would rise above the level, the whole of 

 it was caiTied away as spray by the wind. I saw this occur re- 

 peatedly. The spray and rain made it so dark that only the sur- 

 face of the lake a few feet from the boat could be seen, but as far 

 as could be seen there was violent agitation but no waves, and 

 there was no more rocking of the boat than in a dead calm. This 

 continued for some time, probably an hour after my first observa- 

 tion, when the storm abated, the clouds passed away, and tbe sun 

 came out; but the wind was still blowing a stiff gale from the 

 same direction, and soon waves from two to three feet high 

 caused the little steamer to roll and jump more than was pleas- 

 ant. 



So far as I am aware, it has not been ascertained experimentally 

 what determines the coefficient of friction of air moving over the 

 surface of water. It is obviously this friction which causes waves 

 in the water when the wind blows, and, like all other friction, it 

 doubtless depends measurably on the direction and violence of the 

 impact. But the air, by a force the operation of which is not 

 clearly understood, and which we call evaporation, is constantly 

 pulling out molecules of water and absorbing them in aqueous 

 vapor. 



The units of energy required to transform a pound of water 

 into vapor is a measurable quantity, and according to the law of 

 conservation of energy, the same units of force must be required 

 to do this work, whether tbe temperature be 100° C, or 0° C, or 

 anywhere between. The time in which the work can be done 

 varies with the temperature, but the units of force expended must 

 remain the same. If this is so, the force exerted in evaporation 

 is immense, and its direction, apparently, is from the surface of 

 the water upwards. It must therefore necessarily operate as a 

 resistance when the air moves across the surface of the water at 

 right angles to this direction, and tbus increase friction. 



It seems to be analogous to friction bet ween two solid bodies 

 when one of them absorbs the particles rubbed off from the other ; 

 the absorptiou may not increase the friction, but the rubbing-off 

 does. So in this case, tbe absorption by the air of the molecules 

 or particles of water as aquous vapor may not increase the friction 

 between the air and the water, but pulling them away from the 

 water certainly ought to do it. 



From this it would seem that a dry wind, from its greater ca- 

 pacity to absorb aqueous vapor, ought to produce greater friction 

 and higher waves than a damp wind of the same velocity. This 

 appeared to me to be the case with the winds blowing across Lake 

 Harris at my winter home in Florida. The dry winds following 

 the rain-storm seem to raise higher waves than the damp winds 

 preceding the rain and during its continuance; but without facili- 

 ties or skill for accurately determining either the relative humid- 

 ity or the velocity of the wind, such observations are of no value 

 except to call attention to the subject. 



There is another view of the matter which seems to me to be 

 worthy of examination. If the evaporation-pull when air passes 

 over the surface of a liquid is an element in the resulting friction 

 and consequent wave development, we have, in tke capacity of 

 certain oils to resist evaporation, an explanation of the phenome- 

 non that pouring oil on tbe surface of water diminishes the waves. 

 This is indicated by the fact that kerosene oil, which evaporates 

 rapidly, does not seem to have the effect of diminishing wave- 

 formation. 



Before this probable difference in friction between liquids which 

 evaporate readily and those which resist evaporation had occurred 

 to me I tried the experiment of pouring kerosene oil on the sur- 

 face of Lake Harris when it was very rough and a high wind 

 blowing. It had no perceptible effect in diminishing the waves, 

 but a conscious want of skill in conducting the experiment left 



me in doubt as to whether the failure resulted from that or some 

 other cause. Evaporation takes place more rapidly when the 

 air is not moving, because fresh unsaturated portions of the at- 

 mosphere are being constantly brought into contact with the liquid 

 surface; and the theoretical probability that this evaporation, this 

 pulling-away of molecules of water by the air into itself, is an 

 element in the friction between the wind and the water, is cer- 

 tainly sufficient to justify the labor of its experimental determi- 

 nation. It may be that the thorough saturation of the wind 

 blowing across Lake Eustis, in the case above mentioned, was it- 

 self an element in preventing the wave- formation: the -saturation 

 of the wind may have diminished its friction and consequent ca- 

 pacity for wave development, and the blowing away of incipient 

 waves into spray may have increased the saturation. It was ob- 

 vious that the quantity of water carried off as spray was not at 

 all comparable to that which rises above the level in waves from 

 a wind of less violence. Taking the normal level as the average 

 between the crest and tbe trough of the waves, there was much 

 more water above that level when the lake became rough after 

 the hurricane had passed, than appeared to be carried off in spray 

 while no waves were being formed. 



There is another element of resistance which must be taken into 

 account in determining the friction between wind and water. The 

 air not only absorbs water in the form of aqueous vapor, but the 

 water holds air (or oxygen obtained from it) in solution. This is 

 the air which the fishes breathe, and under atmospheric pressure 

 and near the surface of the water it is estimated that the air thus 

 held in solution constitutes about one-twentieth of the volume of 

 the fluid. This air can be disengaged from the water. It is this 

 disengagement of the air from the water by the suction of a pump, 

 which renders it impossible for a pump to raise ordinary water to 

 the full height to which the atmospheric pressure will raise a 

 column of water from which air is excluded: when the suction of 

 the pump exerts a pull on the water with air in solution sufficient 

 to raise it to about twenty-seven feet, the air in the water is dis- 

 engaged and fills the vacuum chamber, thus stopping the further 

 lifting of the water by the vacuum pull. This disengagement of 

 the air from the water goes on in water-pipes also. In a system 

 of water-pipes the disengaged air collects in the most elevated por- 

 tions of the pipes, and, unless discharged through air-valves, be- 

 comes a serious obstacle to the flow of the water. 



lu ice-making, this air in the water is gotten rid of by distilling 

 the water and recondensing it, or by boiling the water and then 

 freezing it before it has re-absorbed air. If the air is not removed 

 in some way. it remains in the ice in small bubbles, rendering the 

 ice white and porous. 



It is certain that the agitation of water either impedes or facili- 

 tates the absorption of air into solution. The general impression 

 is that agitating the water aerates it, that is, causes it to absorb 

 air; but when water containing odorous vapor is stirred, they are 

 given off. This and some other phenomena seem to me to indi- 

 cate that agitation, while it enables water to retain in solution 

 matter heavier than itself, has the opposite effect with matter 

 lighter than itself, and that the tendency of agitation is to cause 

 water to release gaseous matter held in solution. But whether the 

 agitation of the water tends to cause it to take air into solution, 

 or to release air absorbed when the water was less agitated, the 

 process, either of absorption or of release, probably increases fric- 

 tion between the wind and the water, as the surface of the water 

 becomes agitated by nave-formation. This element of friction 

 must be very small when compared with the far greater work of 

 evaporation, but it ought to be taken into consideration in deter- 

 mining the difference in friction between air and water and air 

 and oil. 



It has been demonstrated, experimentally, that when water 

 evaporates into an- as aqueous vapor the proce.ss goes on by mole- 

 cule after molecule, and not by aggregations of molecules or 

 masses; nor that water absorbs oxygen from the air by taking in 

 each molecule separately; but, according to the accepted theory 

 of diffusion of gases, we must assume that the aqueous vapor re- 

 sulting from evaporation diffuses into the atmosphere by molecules 

 and not by masses; and the fact that the oxygen of the air dis- 

 solved in water is separated from the nili-ogen, the molecular con- 



