October 20, 1905.] 



SCIENCE. 



497 



I 



more nearly in accordance with the laws con- 

 trolling the flow through pipes. These pas- 

 sageways often cross-divide the soil itself into 

 endlessly irregular and varying blocks, so that 

 even deep in the ground both air and water 

 flow in more or less open channels, percolation 

 and transpiration taking place into these so 

 that much of the mass movement of either air 

 or water may occur without passing through 

 capillary spaces. We have called attention to 

 this fact in ' Movements of Ground Water ' ^ 

 and have there shown how far computations 

 based upon laboratory trials may be from what 

 occurs xmder natural conditions. These re- 

 marks apply with especial force to the surface 

 one to four feet of field soils, the movements 

 through which are of greatest agricultural im- 

 portance. In this zone shrinkage cracks and 

 passageways left by the decay of roots or 

 formed by burrowing animals, it appears to 

 the writer, influence in a very profound way 

 the interchange of air as effected through 

 changes of atmospheric pressiire and cause the 

 estimate of the author to be, in our judgjient, 

 very much below the true value. 



The particular mode of action of atmos- 

 pheric pressure which, it appears to us, must 

 be most potent in causing an interchange of 

 air in the surface soil has not been considered 

 in the Bulletin under review. We refer to 

 the pressure and suctional effects which result 

 from, or are associated with, changes in wind 

 velocity and the turbulency of the air move- 

 ment at the earth's surface. The fluctuations 

 of pressure to which we refer are of too short 

 duration to be" recorded by ordinary barographs, 

 but they are nevertheless of sufficient length 

 to be transmitted into the soil and their niagTii- 

 tude often exceeds some of those which the au- 

 thor has considered, while their frequency is 

 very great. The agency which it appears to 

 us is likely to be found most influential in the 

 aeration of the surface soil is the wind itself, 

 as it is the chief factor which effects a change 

 of air in a house. As the air passes over the 

 surface of a field, there must be maintained 

 an excess of pressure on the windward side of 



■' ' Principles and Movements of Ground Water,' 

 XTX. Annual Report, U. S.^ Geol. Survey, Part 

 II., p. 249. 



obstructions to flow large and small of what- 

 ever kind, while on the leeward side there 

 will be maintained a deficiency of pressure, 

 so that on the whole air will be flowing into 

 the soil in some places, traveling more or 

 less horizontally and then rising to come 

 out at places where the air pressure is 

 less. And we do not see how it is pos- 

 sible that this influence can be limited to so 

 small a depth as the author estimates for 

 barometric '■ rinsing.' Besides this, when the 

 wind is blowing strong and is gusty in char- 

 acter there is a turbulency of flow analogous 

 to that which occurs in a stream flowing down 

 a rapid, giving to the air a downward thrust 

 upon the surface, from which it rebounds, 

 driving the air into the soil in some places 

 and sucking it out in others. 



But to these statements the author will 

 doubtless reply that the writer is merely 

 naming possible factors and doing so without 

 testing their probable efficiency even n^athe- 

 matically. This is quite true, but both these 

 and his own views can and should be checked 

 by field observations and he is aware that we 

 had begun a series of observations on the com- 

 position of soil air collected simultaneously 

 at different depths down to four feet and that 

 a considerable amount of the data so obtained 

 are unpublished among the records of the 

 bureau. He is well aware too that my object 

 in having him called to the bureau was that 

 he might make investigations along exactly 

 the lines presented in the Bulletin, with many 

 others, but to have him do so in conjunction 

 with simultaneous field studies so that each 

 line of work would supplement and check the 

 other and be definitely related to observed crop 

 and soil conditions. My criticism now is that 

 the langiiage of the Bulletin conveys the im- 

 pression that such laboratory and mathemat- 

 ical treatment as he has presented have been 

 sufficient to solve the method of soil aeration 

 and to give a measure of the rate at which it 

 occurs under field conditions, without making 

 a field cheek on the results. 



In regard to the longer period atmospheric 

 waves, which the author has specifically con- 

 sidered, attention should be called to the fact 

 that these, even when they are as short as 



