a62 



NA rURE 



\_Dcc. 



and b/ O. T. Sheniiaii (|jre-ent by invitation), on personality in 

 the measures of the diameter of Venus : in mathematics, by 

 S. Newcomb, on the theory of errors of observation, and pro- 

 bible results : in physics, by S. Newcomb, on the use of tlie 

 word " light " ; by \V. U. Brewer, on the subsidence of particles 

 in liquids ; and by H. A. Rowland, on a new photograph of the 

 solar spectrum : in meteorology, by E. Ijoomis, on the reduction 

 of barometric observations to sea-level : in geol igy, by T. S. 

 Hunt, on the Animikie rocks of Lake Superior ; by J. D. Dana, 

 on the stratified drift of the New Haven region ; by B. Silliman, 

 on the mineralogy and litholojy of the B odie mining district ; 

 and l)y J. S. Newberry, on the ancient glaciation of North 

 America: in chemi-try, by W. Gibbs, on phospho-vanadates, 

 arsenio-vanadates, and antimonio-vanadates, and on the existence 

 of new acids of | hosphorus : in physiological chemistry, by 

 R. H. Chittenden (present by invitation), on new primary 

 cleavage forms of albuminous matter : in palaeontology, by J. 

 Hall, on the Pectinida; and Aviculida; of the Devonian system ; 

 and by O. C. Marsh, on the affinities of the dinosaurian reptiles : 

 and in anthropology, by A. G. Bell, on the formation of a deaf 

 variety of the human race ; and by J. \V. Ponell, on marriage 

 institutions in tribal society. 



The report of the Committee on Glucose, appointed by the 

 President in conformity with a request from the Governmen , 

 was accepted by the Academy, and uill be transmitted to Con- 

 gress with tiie President's report. This will also embody the 

 proceedings of recent meetings of the Acidemy, the rep )rt of 

 the Committee on Alcohol, and that on the eclipse of the sun, 

 together with the thinks of the Academy to the Secretary of the 

 Navy and the officers of the Hartford for their cooperation in 

 the expedition to Caroline Island. It will also include an ex- 

 pression of the approval of the Academy of the efforts now 

 making to secure a system of uniform time. 



The next slated session of the Academy will be held in 

 "Washington in April next, and it is prubalile that the following 

 mid-year seFsion will be held in Cambridge. 



RIPPLE-MARKS^ 

 TN the first series of experiments a cylindrical vessel, like a Hat 

 bath, with upright sides, was placed on a table, which was 

 free to turn about a vertical axis. Some fine sand was .strewn 

 over the bottom to a depth o( about an inch, and water was 

 poured in until it stood three inches deep ever the sand. It was 

 found that rotational oscillation with a jerking motion of small 

 amplitude gave rise almost immediately to beautiful radial 

 ripples all round the bath. If the jerks were of small amplitude 

 the ripples were small, and if larger they were larger. The 

 radiating ripples began first to appe.ir at the outer margin of the 

 baih and grew inwards ; but the growth stopped after they had 

 extended to a certain di-tance. If the jerking motion was 

 violent, ri])ples were not formed near the circumference, and 

 they only began at some distance inwards. 



An analysis of the observations was made on the hypothesis 

 that the water remained still, when the bath oscillated with a 

 simple harmonic motion. The problem was to find vihether A, 

 the wave-len'^'th of ripple (in inches) was directly ]iroportional to 

 V, the maximum velocity of the water relatively to the bottom 

 during the oscillatory motion ; also to lind the values of '<-\ and 

 v„. the least and greatest velocities of the water compatible with 

 the formation of ripple-mark. 



It apjiears that, for the particular sand used, r'l is half a foot 

 per second, and v„ a foot per second ; and ihat the wave-length 

 of ripple, A, is •00245!-; when v is measured in inches per minute. 

 The several results were as fairly consistent with one another as 

 could be expected. The hypothesis that the water as a whole 

 executes a simple harmonic oscillation relatively to the bottom is 

 iifit, however, exact, and does not give the maximum velocity of 

 the water in cmtact with the sand relatively thereto. The 

 quantity called v is not in reality the maximum veloc'ty of the 

 water in contact with the bottom relatively thereto, but it is 

 6283 times the amplitude multiplied by the frequency. Thus 

 we cannot conclude that a current of half a foot per second is 

 just sufficient to stir the -and. In the state of oscillation corre- 

 sponding to V, it is probable that part of the water at the bottom 

 is moving with a velocity much greater than half a font per 

 second relatively to the sand. 



' " On ihe rormatir-n nf Ripple-m.irk in Sand." Abstract of a pajier by 

 G H. Darwin, F.K.S. I'lumian Professjr and Fellow of Trinity College, 

 Cambridge, read before the Royal Society on November 22, 1883. 



It was after making these experiments that what appears to 

 be the key-note of the whole phenomenon was discovered. 



A series of ripples extending inwards for some distance having 

 been made by oscillation, and the water having come to rest, 

 the bath was turned slowly and nearly uniformly round. The 

 uniform current flattened the tops of the ripples, but made the 

 lee-side steeper. 



It was conjectured that there would be eddies or vortices on 

 the lee-side, and in fact minute particles lying on the surface of 

 the sand w ere observed to climb up the lee-slope of the ripples 

 apparently againsl stream. This proved conclusively the exist- 

 ence of the suspected vortices. 



If when the hath was at rest a sudden motion was given in 

 one direction, the sand on the lee-side of each ripple was ob- 

 served to be churned up by a vortex. By giving a short and 

 sudden motio)i the direct stream might be seen to pile up the 

 sand on the weather-side and the voitex to pile it up on the lee- 

 side. The sand so displaced formed two little parallel ridges, 

 that on the lee-side being a little below the crest of the ripple- 

 mirk. 



For the purpose of examining the vortices a glass tube was 

 drawn out to a fine point and fitted at the other end with a short 

 piece of india-rubber tube. With this a drop of ink could be 

 squirted out at the bottom of the water. This method was 

 adopted in all sub equent observations, and it proved very 

 valuable. It may be worth mentioning that common ink, which 

 is heavier than water, was better than aniline dye. 



A drop of ink was placed in the furrow between two ripples : 

 as soon as the continuous stream passed, the ink was parted into 

 two portions, one being sucked back apparently against stream 

 up the lee-side of the ripple-mark, and the other being carried 

 by the direct stream towards the crest. These points being 

 settled, it remained to discover how the vortices were arranged 

 which undoubtedly must exist in the oscillatory formation of 

 regular ripples. 



The observations v/ere made in two ways, first with a glass 

 trough so arranged that it could be gently rocked by band, and 

 secondly with an ocillaiing sheet of glass. 



When the trough is half fillel with water, and sand is 

 sprinkled on the bottom, it is easy to obtain admirable ripple- 

 marks by gently rocking the trough. 



When a very small quantity of sand is sprinkled in and the 

 rocking begins, the sand dances backwards and forwards on the 

 bottom, the grains rolling as they go. 



Very shortly the sand begins to aggregate into irregular little 

 floccnlent masse-, the appearance being something like that of 

 curdling milk. The position of the masses seems to he solely 

 determined by the friction of the sand on the bottom, and as 

 soon as a grain sticks, it thereby increases the friction at that 

 place. 



The aggregations gradually become elongated and rearrange 

 themselves. As soon as the formation is definite enough to 

 make the measurement of the wave-length possible, it is found 

 that the wave-length is a'lout one-half of what it becomes in the 

 ultimate formation. 



Some of the elongated patches di-appear, and others fuse 

 together and form ridges, the ridges then become straighter, and 

 finally a re^jular ripple-mark is formed, with the wave-length 

 double that in the initial stage. 



If, after the formation of regular ripples, and the deposition of 

 a drop of ink at the bottom, a very gentle oscillation be started, 

 the layer of ink on the crest of a ripple becomes thicker and 

 thinner alternately, swaying backwards and forwards; then a 

 little tail of ink rises from the crest, and the point of growth 

 oscillates on each side of the crest ; the end of the tail flips 

 backwards and forwards. Next the end of the tail spreads 

 out laterally on each side, so Ihat a sort of mushroom of 

 ink is formed, the stalk of the mushroom dancing to and 

 fro. The height of the mushroom is generally less than a 

 millimetre. 



The elongated hollows under the mushroom are the centres of 

 vortices, and the stem is the upward current. If the ink be 

 thick, these spaces are clouded, and the a]>pearance is simply 

 that of an alternate thickening and thinning of the ink on the 

 crest. The oscillations being still gentle, but not so gentle as at 

 first, streams of ink from the two mushrooms on adjacent crests 

 creep down the two slopes into the furrow br-tween the adjacent 

 ridges, and where they meet a column of ink begins to rise from 

 the part of the water w hose mean ])osition is in the centre of the 

 furrow. r 



