Sept. 1 8, 1884] 



NA TURE 



487 



the level paved bed presents the appearance of a ladder of low 

 advancing waves occurring at regular intervals of about 40 

 feet over the lower slope of one in twelve, and at less regular 

 intervals of about 12 feet over the steeper slope of one in 

 nine. 



Of the motion of the stream over the lower slope of one in 

 twelve the following particulars were noticed : — 



A floating body travels at the rate of 9i feet per second, 

 but this does not represent the speed of any part of the water. 



/Vave 3 Wave 2 Wave I 



\^Vr^^#^lV^H^wf 



GROUND PLAN 



Scale 32 feet to 1 inch 



The wave-heads advanced at the rate of 13 feet a second, and 

 the intervening stretches of stiller water (as nearly a-. I could 

 judge) at about 6 feet a second. It is evident that the upper 

 and lower currents are travelling at different rates — the bottom 

 current retarded by friction, the surface current advanced over 

 it by gravitation, accumulating at intervals of about 40 feet into 

 wave-heads of a semicircular form, the sides being bent back by 

 latent friction. 



The motion of a floating body in the stream of advancing 

 waves is very peculiar. A piece of wood thrown in at A, 

 just in front of the advancing wave, No. 1, is for a moment 

 carried forward by it, but the slower lower stratum gains the 

 mastery, and the wave advances in front of the wood, which is 

 successively found at B, c, D, E, &c. relatively to the advancing 

 wave-heads, the floating wood recedes up the stream, though 

 actually advancing at a rate between that of the upper and 

 under or ground current. 



The waves occur at intervals of about 40 feet, and occupy a 

 trifle over 3 seconds in passing over the space that separates 

 them. 



Of the motion of the stream over the steeper slope of about 

 one in nine, the following particulars were noticed : — 



A floating body travels at the rate of 12^ feet per second. The 

 wave-heads were less clearly defined than on the less steep 



ACTION Slope n, " 



^fc^iil 



': 



c.rJOUND PLAN 



Scale 32 feet to 



incline, and it was difficult to accurately measure their rate of 

 advance, but as in the other case they rapidly overshot a floating 

 piece of wood. They occur at much shorter intervals (about 

 12 feet) than on the less steep incline. George Maw 



Interlaken, June 28 



I may mention that my observations referred to by Mr. Maw 

 were made upon the current of the River Severn with a view to 



explain the cause why the men who navigate the barges, in 

 descending this river by the force of the current only are 

 enabled to steer with a moderate degree of effectiveness. The 

 power results from the different velocities of the current at and 

 beneath the surface. A little below the surface, roughly speak- 

 ing at about one-fifth of the actual depth, the current seems to 

 have its maximum velocity, and consequently the hull of the 

 vessel floating down the stream is immersed in water flowing 

 more rapidly than that at the surface, on which the rudder for 

 the most part acts. 



I was enabled to demonstrate this fact by the following simple 

 experiment. Having noticed that leaves of trees, after lying for 

 some time on the ground and nearly saturated with .water, be- 

 come almost of the same, and after a longer time of greater, 

 specific gravity than water, it occurred to me that such leaves, 

 while in the first-named stage, would show what I desired to 

 know, namely, the relative velocities of the stream at different 

 levels below its surface. Two straight bars of wood, each about 

 thirteen or fourteen feet long, were tied together at one end, 

 between the two the foot-stalks of a number of poplar leaves 

 were inserted (this kind was chosen because of the length of the 

 footstalk for insertion between the bars, and its brightness of 

 colour rendering it more visible in the depth of the water) ; the 

 bars were charged with the leaves at intervals of about three 

 inches, and then, choosing a place where the river was of suit- 

 able depth, the bars charged with leaves were plunged into the 

 water, the connected ends touching the ground. The water was 

 so clear that every leaf remained visible ; then I opened the 

 ends of the bars at the surface, and was gratified by seeing 

 every leaf floating away and preserving as to depth very nearly 

 the same relative position. Floating with the stream in my 

 boat, I soon saw those nearest the bottom gradually lagging 

 behind, and still more was I gratified when, after proceeding 

 about forty yards, the leaves that were about two feet below the 

 surface had distanced those at the surface in an unmistakable 

 manner by at least three feet, the current being about four feet 

 per second. The whole series forming a curve as is here 

 shown. 



Greatly pleased with this first experiment, I was not satisfied 

 till I had repeated it again and again, not only on that occasion, 

 but when the wind was blowing down the river, and therefore 

 should have accelerated the leaf at the surface, which it un- 

 doubtedly did ; but only the leaf on the surface, and that to a 

 much smaller degree than I expected, and it left unaffected all 

 that were beneath. A calm day is the best for this experiment, 

 because the ripple renders it difficult to see below the surface. 

 The water must of course be clear, a condition with which we 

 are much favoured in this river. Mr. Maw's observations of the 

 different velocities of the pieces of wood and the wave heads are 

 quite in harmony with mine ; the depth of the water in the 

 stream at Merligen would be only a few inches, and pieces of 

 wood were immersed so deeply that they would be more affected 

 by the retarded current four-fifths below than by that one-fifth at 

 the surface. J. P. G. Smith 



Sweyney Cliff, Coalport, Shropshire- 



Ocean Swells 



The late melancholy accident in Fingal's Cave, Staffa, by 

 which three lives were lost, when several visitors to the island 

 were washed off the railed ledge by a large wave which suddenly 

 and unexpectedly broke into the cave, leads me to submit the 

 following account of a somewhat similar wave and on the same 

 part of the coast. 



On the 4th inst. I took a small 5-ton sailing-boat from Oban 

 to the Island of Lismore. We had a steady south-west breeze, 

 going there with an even slight swell in the more open part, 

 coming up the Firth of Lorne from the Atlantic. On our return 

 the wind dropped to a dead calm and shifted to the south-east- 

 ward, so that to get back we took to the oars, the water becoming 

 perfectly smooth as we neared Kerrera (between 5 and half-past 

 5 o'clock), when, standing at the bow, and looking seaward, I 

 was surprised to see a broad wave or long swell coming from 

 the south-westward, followed by two minor undulations. They 



