OCTOBEK 17, 1913] 



SCIENCE 



555 



sion, and advisory engineer, commission for 

 the equitable distribution of the waters of the 

 Eio Grande, takes up tbe problem of bow 

 much space a given weight of river-borne silt 

 will occupy when deposited in a reservoir, 

 saying, on pages 11 and 12: 



It was evident that the per cent, of bulk, ob- 

 tained from test tubes, would be too large for the 

 desired unit because there was no weight on the 

 silt in the tube to compact it, as there would be 

 in a reservoir. . . . 



Something more than guesswork was wanted. 

 It did not seem proper to us to found all our silt 

 calculations on an assumed bulk for it which was, 

 as it were, simply pulled out of the air. The de- 

 sire was to approximate as closely as possible to 

 the conditions which would be found in the bot- 

 tom of a reservoir. After considering various 

 schemes, to all of which there seemed to be valid 

 objections, it was finally decided to seek a mud 

 bar in the river where the water had been com- 

 paratively still and which had shrunk enough to 

 show material cracks, and to cut from this bar a 

 three-inch cube, have it dried out and weighed 

 and to abide by the result, whatever it was. The 

 idea was that a bar should be chosen which had 

 shrunken enough to make up for the compression 

 which the silt in the bottom of a reservoir would 

 undergo from the weight of the water over it. 

 Of course, the necessary amount of shrinkage 

 could not be told exactly, but it was thought that 

 a fairly good guess could be made. 



The three-inch cube was ccillected, dried 

 and found to weigh 85 per cent, as much as a 

 three-inch cube of water. It was, therefore, 

 assumed that " the above experiment fairly 

 determined the weight of reservoir silt and 

 that all silt determinations should be divided 

 by 0.85 in order to obtain the actual final vol- 

 ume of the silt." The collection of the three- 

 inch cube of silt is further described on page 

 Y5 of the report. 



The first idea, which seems incorrect, is that 

 deep water through its greater weight makes 

 deposited silt more compact than shallow 

 water. If the pores are filled with water, the 

 pressure must be equal in all directions and 

 the individual particles of silt being practi- 

 cally incompressible, the weight of the water 

 must have negligible effect on the compact- 

 ness of the silt. If the pores are not filled 



with water, but contain some air or other gas, 

 the material would be compressed in propor- 

 tion to the quantity of gas and the amount of 

 pressure, but it does not seem probable that 

 the compactness of silt is, in general, greatly 

 affected by compression of included gases. It 

 seems more reasonable to suppose that any 

 greater compactness displayed by silt de- 

 posited in deep water is due to the arrange- 

 ment of the particles or a modification of 

 their form, brought about by the great dis- 

 tance traversed in settling, and especially is 

 this true unless it can be shown that such silt 

 expands when taken out of the water. 



The second somewhat surprising idea is 

 that one three-inch cube furnishes a better 

 basis for determining the specific gravity of 

 Eio Grande silt than all other available data, 

 both inferential and experimental. If this be 

 correct, there is certainly great need of adding 

 to the available data, for the determination 

 concerning the three-inch cube seems to be a 

 small foundation for the argument and hun- 

 dreds of computations which are based upon 

 them. The result obtained, namely, that silt 

 free from water weighs only 53 pounds per 

 cubic foot, is considerably below most esti- 

 mates and means that the material has a pore 

 space of nearly 68 per cent. '■^^ w. Shaw 



ON FSYCBOLOGT AND MEDICAL 

 EDUCATION-' 

 Following the symposium on psychology 

 1 Report of the Committee of the American Psy- 

 chological Association. The committee was con- 

 stituted as follows: Shepherd Ivory Franz, scien- 

 tific director and psychologist, Government Hos- 

 pital for the Insane, and professor of physiology, 

 George Washington Medical School, chairman; 

 E. E. Southard, professor of neuropathology, Har- 

 vard Medical School, and director of the psycho- 

 pathic department of the Boston State Hospital, 

 and J. B. Watson, professor of psychology and 

 director of the psychological laboratory, Johns 

 Hopkins University. The scope of the inquiries 

 of the committee was determined by the commit- 

 tee ; the present report was written by the chair- 

 man, who is responsible for its form and the 

 accuracy of its parts, but all the members of the 

 committee are in accord with the conclusions. 



