THE OBSERVATIONS. 



27 



explained in the same manner, by appeal to up- 

 ward movement of filaments of current, but 

 the recent studies have led me to entertain 

 a different view. Before this view is presented 

 an account will be given of certain observations 

 which were made with the use of the trough 

 having glass sides. 



Through the trough was passed a current 

 transporting sand of uniform grade, and the 

 conditions were such that the sand bed and 

 water surface were smooth. In the same water 

 floated a few fine particles and thin flakes of 

 mica, illustrating suspension, but there was no 

 intergradation of the two processes. Viewed 

 from the side, the saltation was seen to occupy 

 at the bottom of the current a space with a 

 definite upper limit, parallel to the sand bed. 

 Within this space the zone of saltation the 

 distribution of flying grains was systematic, 

 the cloud being dense below and thin above, 

 but not perceptibly varying from point to 

 point along the bed. Viewed from above, the 

 surface of the cloud seemed uniform and level, 

 and it all appeared to be moving in the same 

 direction. There was no suggestion of swirls 

 in the current. 



When, in looking from the side, attention 

 was directed to the base of the zone, it was 

 easy to watch grains that traveled half by 

 rolling and half by skipping, and these moved 

 quite slowly; but higher in the zone the 

 motions were so rapid and diverse that all was 

 a blur. To resolve this blur a sliding dia- 

 phragm was arranged. This consisted of a 

 short board with a hole in it. The board hung 



FIGURE 4. Diagrammatic view of part of experiment trough with glass 

 panels (A ) and sliding screen (B). C, Hole in screen. 



outside the wall of the trough, being supported 

 by a cleat above in such manner that it could be 

 slid along the trough. (See fig. 4.) The hole, 

 about 2 niches square, gave a restricted view of 

 the saltation zone. By sliding the board in the 

 direction of the current and keeping the eyes 

 opposite, a traveling field of view was obtained. 

 Manifestly if the field traveled at the same rate 

 as the current, any object moving with the 

 current would appear at rest to the observer, 

 because there would be no relative motion of 



observer and object; and if objects in the water 

 were moving (horizontally) at different rates, 

 those coinciding hi rate with the field would be 

 seen as if at rest, while the others would be seen 

 as moving. 



When the field was moved slowly the rolling 

 grains ceased to be distinct but were replaced 

 in distinctness by grains that seemed to bob 

 up and down. These vibrated through a space 

 of two or three diameters, as if repeatedly 

 striking the bed and rebounding. In inter- 

 preting this appearance, allowance must be 



FIGURE 5. Appearance of the zone of saltation, as viewed from the side 

 with a moving fleld. 



made for the fact that the grains were dis- 

 tinctly seen because they were moving hori- 

 zontally about as fast as the diaphragm. Their 

 paths were really low-arching curves, and only 

 the vertical factor remained when the hori- 

 zontal was abstracted. It is probable also that 

 the appearance of rebounding was largely 

 illusory, most of the grams either stopping at 

 the end of the leap, or else leaping next time 

 with a different velocity. 



When the field was moved somewhat faster, 

 the bobbing grains disappeared and there came 

 into distinct vision a set of grains quite free 

 from the bed and occupying a belt within the sal- 

 tation zone. All the zone above and below them 

 was blurred. In the middle of the belt vertical 

 motion was to be discerned but wa* less con- 

 spicuous than in the lower zone. Where dis- 

 tinctness graded into blurring, lines of motion 

 could be seen which were oblique and curved, 

 the lines above the belt curving forward and 

 those below backward, as shown in figure 5. 



With progressively faster motion of the field 

 the belt of distinct vision rose higher, until the 

 top of the zone was reached, when all the lower 

 part was blurred. 



The systematic gradation of velocity and 

 other features from the bed upward and the 



