NA TURE 



[May 22, iJ 



minerals, more or less altered. The Foraminifera and fragments 

 of Echinoderms and other organisms in these muds a.e frequently 

 filled with glauconitic substance, and beautiful casts of these 

 organisms remain after treatment with weak acid. At times 

 there are few calcareous organisms in these deposits, and at other 

 times the remains of Diatoms and Radiolarians are abundant. 

 When these muds are dried they become earthy and of a gray- 

 green colour. They frequently give out a sulphuretted hydrogen 

 odour. The green colour appears sometimes to be due to the 

 presence of organic matter, probably of vegetable origin, and to 

 the reduction of peroxide of iron to protoxide under its influence. 

 The green sands differ from the muds only in the comparative 

 absence of the argillaceous and other amorphous matter, and by 

 the more important part played by the grains of glauconite, 

 which chiefly give the green colour to these sands. 



Red Muds. — In some localities, as for instance off the Bra- 

 zilian coast of America, the deposits differ from blue muds by 

 the large quantity of ochreous matter brought down by the rivers 

 and deposited along the coast. The ferruginous particles when 

 mixed up with the argillaceous matter give the whole deposit a 

 reddish colour. These deposits, rich in iron in the state of 

 limonite, do not appear to contain any traces of glauconite, and 

 have relatively few remains of siliceous organisms. 



Volcanic Muds and Sands. — The muds and sands around vol- 

 canic islands are black or gray ; when dried they are rarely 

 coherent. The mineral particles are generally fragmentary, and 

 consist of lapilli of the basic and acid series of modern volcanic 

 locks, which are scoriaceous or compact, vitreous or crystalline, 

 and usually present traces of alteration. The minerals are 

 sometimes isolated, sometimes surrounded by their matrix, 

 and consist principally of plagioclases, sanadin, amphibole, 

 pyroxene, biotite, olivine, and magnetic iron ; the size of the 

 particles diminishes with distance from the shore, but the mean 

 diameter is generally o'5 mm. Glauconite does not appear to 

 be present in these deposits, and quartz is also very rare or 

 absent. The fragments of shells and rocks are frequently 

 covered with a coating of peroxide of manganese. .Shells of 

 calcareous organisms are often present in great abundance, and 

 render the deposit of a lighter colour. The remains of Diatoms 

 and Radiolarians are usually present. 



Coral Muds. — These muds frequently contain as much as 95 

 per cent, of carbonate of lime, which consists of fragments of 

 Corals, calcareous Alga:', Foraminifera, Serpulae, Mollusks, and 

 remains of other lime-secreting organisms. There is a large 

 amount of amorphous calcareous matter, which gives the deposit 

 a sticky and chalky character. The particles may be of all sizes 

 according to the distance from the reefs, the mean diameter 

 being I to 2 mm., but occasionally there are large blocks of 

 coral and large calcareous concretions ; the panicles are white 

 and red. Remains of siliceous organisms seldom make up over 

 2 or 3 per cent, of a typical coral mud. The residue consists 

 usually of a small amount of argillaceous matter, with a few 

 fragments of feldspar and other volcanic minerals ; but off 

 barrier and fringing reefs facing continents we may have a great 

 variety of rocks and minerals. Beyond a depth of 1000 fathoms 

 .ill coral islands the debris of the reefs begins to diminish, and 

 the remains of pelagic organisms to increase; the deposit be- 

 comes more argillaceous, of a reddish or rose colour, and gra- 

 dually passes into a Globigerina ooze or red clay. Coral Sands 

 contain much less amorphous matter than coral muds, bul in 

 other respects they are similar, the sands being usually found 

 nearer the reefs and in shallower water than the muds, except 

 inside lagoons. In some regions the remains of calcareous algae 

 predominate, and in these cases the name era. 'line mud or sand 

 i, employed to point out the distinction. 



Such is a rapid view of the deposits found in the deeper waters 

 of the littoral zones, where the debris from the neighbouring 

 land plays the most important part in the formation of muds and 

 ;ands. 



When, however, we pass beyond a distance of about 200 

 mile- from land, we find that the deposits ace characterised 

 by the great abundance of fragmentary volcanic materials which 

 have usually undergone great alteration, and by the enormous 

 abundance of the shells and skeletons 1 if minute pelagic organisms 

 which have (alien to the bottom from the surface waters. These 

 true deep-sea deposits may be divided into those in which the 

 organic elements predominate, and those in which the mineral 

 constituents play the chief part. We shall commence with the 

 former. 



( To l>e continued. ) 



THE TWO MANNERS OF MOTION OF 

 WA TER l 

 T T has long been a matter of very general regret with those 

 who are interested in natural philosophy that in spite of the 

 most strenuous efforts of the ablest mathematicians the theory of 

 fluid motion fits very ill with the actual behaviour of fluids, and 

 this for unexplained reasons. The theory itself appears to be 

 very tolerably complete, and affords the means of calculating the 

 results to be expected in almost every case of fluid motion, but 

 while in many cases the theoretical results agree with those actu- 

 ally obtained, in other cases they are altogether different. 



If we take a small body, such as a raindrop, moving through 

 the air, the theory gives us the true law of resistance ; but if we 

 take a large body, such as a ship moving through the water, the 

 theoretical law of resistance is altogether out ; and what is the 

 most unsatisfactory part of the matter is that the theory affords 

 no clue to the reason why it should apply to the one class more 

 than to the other. 



When seven years ago I had the honour of lecturing in this 

 room on the then novel subject of vortex motion, I ventured to 

 insist that the reason why such ill success had attended our theo- 

 retical efforts was because, owing to the uniform clearness or 

 opacity of water and air, we can see nothing of the internal 

 motion, and while exhibiting the phenomena of vortex rings in 

 water, rendered strikingly apparent by partially colouring the 

 water, but otherwise as strikingly invisible, I ventured to predict 

 that the more general application of this method, which I may 

 call the method of colour-bands, would reveal clues to those 

 mysteries of fluid motion which had baffled philosophy. 



To-night I venture to claim what is at all events a partial 

 verification of that prediction. The fact that we can see as far 

 into fluids as into solids naturally raises the question why the 

 same success should not have been obtained in the case of the 

 theory of fluids as in that of solids. The answer is plain enough. 

 As a rule there is no internal motion in solid bodies, and hence 

 Miir theory, based on the assumption of relative internal rest, 

 applies to all cases. It is not, however, impossible that an at 

 all events seemingly solid body should have internal motion, and 

 a simple experiment will show that if a class of such bodies 

 existed they would apparently have disobeyed the laws of motion. 



These two wooden cubes are apparently just alike, each has a 

 string tied to it. Now if a ball is suspended by a string you 

 all know that it hangs vertically below the point of suspension, 

 or swings like a pendulum ; you see this one does so, the other 

 you see behaves quite differently, turning up sideways. The 

 effect is very striking so long as you do not know the cause. 

 There is a heavy revolving wheel inside which makes it behave 

 like a top. 



Now what I wish you to see is that had such bodies been a 

 work of Nature so that we could not see what was going on — if, 

 for instance, apples were of this nature while pears were what 

 they are, the laws of motion would not have been discovered, or 

 if discovered for pears would not have applied-to apples, and so 

 would hardly have been thought satisfactory. 



Such is the case with fluids. Here are two vessels of water 

 which appear exactly similar, even more so than the solids, 

 iu can see right through them, and there is nothing 

 unreasonable in supposing that the same laws of motion would 

 apply to both vessels. The application of the method of colour- 

 wever, reveals a secret — the water of the one is at rest 

 while that in the other is in a high state of agitation. 



I am speaking of the two manners of motion of water — not 

 because there arc only two motions possible : looked at by their 

 general appearance the motions of water are infinite in number ; 

 Imt what 11 is my object to make clear to-night is that all the 

 various phenomena <>f moving water may be divided into two 

 broadly distinct classes, not according to what with uniform 

 fluids are their apparent motions, but according to what are the 

 internal motions of the fluids which are invisible with clear fluids 

 but wliich become visible with colour-bands. 



The phenomena to be shown will, I hope, have some interest 

 in themselves, but their intrinsic interest is as nothing compared 

 to their philosophical interest. On this, however, I can but 

 slightly touch. I have already pointed out that the problems of 

 fluid motion may be divided into two classes, those in which the 

 theoretical results agree with the experimental and those in which 

 they are altogether different. Now what makes the recognition 



1 Lecture at the Royal I 

 F.R.S. 



1 Friday, March 2S, by Prof. Osbo 



