622 



NATURE 



[July 14, 192 1 



tion of the presence of these bodies. One or two 

 writers have recorded the occurrence of sacs with 

 watery contents in different molluscs. The most 

 notable instance known to me is that of Modiola 

 modiolus, which, in the Barrow Channel, opposite the 

 Lancashire and Western Sea F"isheries Laboratory at 

 Piel, frequently contains leathery periostracum pearls 

 in the mantle margin, and, associated with these, 

 cysts lined with epidermis, containing" watery or 

 mucoid matter. In one of these cysts, some twenty 

 years ago, I found what appeared to be the spores of 

 a protozoon of some kind, but I have not been able to 

 repeat this observation. If sacs of this kind, whether 

 of parasitic origin or due to some pathological con- 

 dition of the oyster not of parasitic origin, occurred in 

 the Ceylon pearl oyster, and either occasionally burst 

 or normally dehisced to liberate a parasite or its 

 spores, such bodies as small grains of sand, or (as 

 in one of the pearls figured by me) a small quantity 

 of mud containing diatoms, etc., might sometimes be 

 swept into the sacs by the ciliary current and become 

 the "nuclei " of pearls. 



The distribution of pearl-producing examples of the 

 various species of molluscs points to the conclusion 

 that the presence of pearls — in other words, the de- 

 velopment in the tissues of the mollusc of pearl-sacs — 

 is associated either with parasites which are peculiar 

 to certain localities, or with pathological conditions, 

 following upon particular environmental conditions, 

 which are strictly local in their occurrence. Thus the 

 Ceylon pearl oyster, which produces pearls abun- 

 dantly in the Gulf of Manar, rarely produces them 

 in Trincomalee Harbour, while the distribution of 

 pearl-producing: beds of Margaritifera maxima and 

 M. margaritifera is still more striking. We find the 

 same local distribution of pearl-producing individuals 

 in the ft-esh-wa^er nearl mussel Margaritana, and more 

 noticeably in .^nodonta. 



Personally I am inclined to anticipate that in man.y 

 of these cases pearl formation will yet be shown to 

 be associated with unicellular parasites. But, whether 

 the pearl-sac is of parasitic origin, or due to some 

 obscure response of the mollusc to a particular set of 

 environmental conditions, it might well prove a highly 

 profitable enterprise to transplant yount* examples, 

 particularlv of such species as Margaritifera maxima 

 and M. margaritifera from beds where the per- 

 centage of pearl production is low, or where pearls 

 are never produced, to some of those beds where 

 almost every individual contains pearls. This process, 

 if successful, would bring the production of pearls 

 into line with the relaying of edible oysters on 

 grounds where the conditions are such as to secure 

 that they will fatten properly for market. 



H. LysTER Jameson. 



Sources and Sinks. 



Mr. Dufton's experiment (N.iture. June 23, 

 p. 522) showing attraction between a source and an 

 equal sink illustrates forcibh' a remark by Mr. A. 

 Mallock in the Issue for August 19, 1920, p. 777 : " In 

 most problems relating to the actual phenomena ex- 

 hibited by fluids in motion, the simple assumptions on 

 which the hydrodynamical theory of text-books rests 

 are insufficient, and experiments are required." At 

 my suggestion Mr. R. Schlapp has recently been 

 making some experiments on the forces between sources 

 and sinks. The vertical limb (about 80 cm. In length) 

 of a T-shaped glass tube dipped into a tank of water, 

 and the horizontal portion rested on V supports. One 

 end of this horizontal part was sealed, the other was 

 connected by rubber tubing either to a high-pressure 

 water supph' or to a water pump, so that the end of 

 the tube in the tank acted as either a source or a sink. 



NO. 2698, VOL. 107] 



Three types of orifice were used : (a) the open end 

 of the glass tube (internal diameter 0-4 cm.) — this 

 worked well as a sink, but was unsatisfactory as a 

 source ; (b) a hollow brass sphere (diameter 2 cm.) 

 with numerous perforations ; (c) a short length of 

 rubber tube having the lower end plugged and per- 

 forations over about 2 cm. On the whole the last 

 arrangement proved the most convenient, but care 

 had to be taken to ensure that no movement arising 

 from lack of symmetry in the size and spacing of the 

 perforations took place when using an isolated source. 



When a single source was in the neighbourhood of 

 a fixed vertical wall, attraction was observed. The 

 attraction Was very distinct at small distances, even 

 with a small flow of water. At greater distances and 

 with a stronger source the motion was irregular. 

 .\ttraction was found also between a sink and a wall. 



When two sources were employed it appeared as if 

 they were under the influence of two forces, one 

 attractive and the other repulsive, the former being 

 predominant at distances less than about 2 cm. At 

 such small distances the sources were drawn together 

 and remained in contact as long as the water flowea. 

 .'\dditional evidence for the existence of a repulsive 

 force was afforded by the observation that a fixed 

 source repelled a second tube through which no water 

 was flowing with a force which was greater or less 

 according as the flow of water was large or small ; 

 but at small distances the action was attractive. Two 

 sinks attracted one another, no repulsive tendency 

 being observed. 



Although Mr. Dufton's experiment showing apparent 

 attraction between a source and a sink in a Win- 

 chester bottle was repeated successfully, experiments 

 in an open tank, using the perforated rubber tube as 

 a source and a similar arrangement or an open 

 tube as a sink, showed strong repulsion between 

 source and sink. 



It is, of course, obvious that the conditions in 

 such experiments differ in several respects from those 

 assumed In the hydrodynamical theory of sources and 

 sinks in an infinite mass of fluid. H. S. .Allen. 



The University, Edinburgh. 



Helicopters. 



Mr. Mallock, in his letter in Nature of June 30, 

 p. 553, omits the chief reasons for the non-success of 

 helicopters so far. 



The first and, to the engineer, most obvious diffi- 

 culty Is the extra weight of moving as compared with 

 fixed wings, and this applies to ornlthopters equally. 



The second, demonstrated conclusively by Ria- 

 bouchinsky at the Koutchino laboratory In 1909, and 

 recently rediscovered by" ourselves, lies in the 

 phenomenon of mutual and self-Interference of the 

 blades of an airscrew, now commonly called the 

 cascade effect. 



Each blade blows down the next following In the 

 spiral path, then the other blades in turn, then again 

 itself and the others, the effect becoming fainter as 

 the axial distance from the "image" of itself and 

 the others becomes greater. 



In aeroplanes and helicopters, as In all structures 

 which are kept geometrically similar, the weight in- 

 creases as the cube and the lifting surface as the 

 square of the typical dimension, and though some 

 fining down of large structures can be made In com- 

 parison with small, this physical law limits the size 

 alike of the vulture, the elephant, the whale, and the 

 aeroplane. In helicopters the limit comes sooner 

 than in the aeroplane, for the two reasons given above. 



If this fundamental relation is Ignored, the aero- 

 plane or helicopter will be fortunate If It meets no 



