NA TURE 



[June 



measurement of the resistance of the human body, suggested that 

 the latter instrument was too sensitive, and that from self-induction 

 perfect silence could not be obtained. Both these remarks are 

 true ; but if lime and the chairman had permitted, 1 should have 

 said that absolute silence is rarely got, but that the minimum of 

 sound is so easy, after a little practice, to estimate, that one- 

 hundredth of a revolution on either side of it is instantly de- 

 tected. The bridge wire takes ten turns on the barrel ; ci in- 

 sequently this amount is the thousandth part of a wire three 

 metres long. Using a fixed resistance of loo", the possible 

 error is quite unimportant, and even with iooo" it is far within 

 other instrumental accidents. 



But as in the somewhat similar case of counting "beats" 

 between tuning-forks, a sensitive and an educated ear is needed. 

 At first starting I found that 1 made considerable mistakes, 

 one of which is recorded in a paper contributed to Nature 

 some weeks back. W. H. Stone 



Wandsworth 



Simple Methods of Measuring the Transpiration of 

 Plants 



The "potelometre" described in Nature, May 22, p. 79, ap- 

 pears to be an ingenious but a rather complicated instrument. Ex- 

 perience has, however, taught me that the extremest simplicity is 

 most desirable. Mr. Ward hints at difficulties of manipulation 

 which are quite conceivable. The plan I have adopted, and find 

 to answer, as far as il goes, is to insert the cut end in a small 

 test-tube and cover the surface of the water with a little oil. The 

 whole can then be weighed to three places of decimals, and the 

 absolute amount of loss in a given time is easily ascertainable. 



But a serious objection must be made against all experiments 

 with cut shoots and leaves, for they can only give, at best, un- 

 satisfactory results. The amount of transpiration varies so much 

 under the ever-changing conditions of light, heat, dryness, &c, 

 that it is only by a long series of comparative experiments with 

 the same specimen that the differences peculiar to each kind of 

 plant can be ascertained ; and no cut shoot can be employed for 

 two or three days, much less for several clays, as are necessary 

 for obtaining satisfactory results ; as the amount of loss steadily 

 decreases till death ensues, although the shoot may be apparently 

 quite healthy for a long time. I have been experimenting for 

 several summers on the transpiration of plants under coloured 

 lights, and at first used cut specimens, as so many experimenters 

 have done, but I found they were most untrustworthy. I now 

 grow the plants in miniature pots, which are covered up in 

 gutta-percha sheeting. These can be weighed to two places of 

 decimals. By this simple method all difficulties are entirely 

 obviated. George Henslow 



Drayton House, Ealing 



Worm-eating Larva 



The following note, which I received from the Rev. Robt. 

 Dunn of Cricklade, may be worth publishing in reference to Prof. 

 McKenny Hughes's " Notes on Earthworms." Mr. Dunn says : 

 " This afternoon (May 6) on a gravel path I saw a worm 

 wriggling in an unusual way, and stooping down I saw that a 

 big earthworm had a smaller worm hanging on at the belt or 

 knob, or whatever you call it ; so I got a bit of stick and pushed 

 off the parasite and found it no worm, but I should say a sort of 

 centipede, with a very red head, about one inch long. Si 1 I 

 captured him and put him in methylated spirit, when he 

 vomited what I presume was worm's blood." He further adds 

 that what the beast vomited was a stream of crimson fluid ; it 

 separated at once into white flocculent matter with brick-red 

 specks, but since it has all turned into a white sediment. Mr. 

 Dunn sent me the animal, which proves to be the larva of a 

 beetle, either one of the Staphylinidfe or Geodephaga. 1 



Southampton W. E. Darwin 



Cultivation of Salmon Rivers 



I hope we may assume, from the paragraph which appears 

 among the "Notes "in your issue of last Thursday (p. 129), 

 that the Fishery Board for Scotland is about to take some active 

 course towards the removal of obstructions to the ascent of 



1 Mr. W. F. Blaudford has called my attention to an account of a simitar 

 encounter between a worm and a larva given in Dallas's "Elements of Ento- 

 mology," p. 6. 



salmon up Scottish rivers. When you say the Board "is specially 

 desirous to introduce as soon as possible a fishway at the falls, 

 and this, when done, would open up some 500 miles of 

 excellent fishing and spawning ground," I hardly think you can 

 be alluding to any one particular river. Am I correct in sup- 

 posing you refer to the aggregate mileage of rivers in Scotland 

 now closed by natural obstructions, i.e. waterfalls? The Report 

 of the Special Commission to inquire into the condition of the 

 salmon fisheries of Scotland, published in 1871, informed us 

 that the River Tay alone had some 115 miles of river blocked 

 against the salmon by the two natural obstructions of the Tum- 

 mel Falls and the Falls of Garry on the two important Tay 

 tributaries from which the respective waterfalls are named. If 

 your "Note "meant to include the entire mileage of Scottish rivers 

 seriously affected by artificial dams of a more or less obstructive 

 character (and their name is legion in Scotland), as well as by the 

 natural barriers that occur, I think 500 miles of obstructed fish- 

 ing and spawning ground is far too low an estimate ; it might in 

 fact, I should say, be multiplied at the very least by three. Now 

 that theoretical playthings are being laid aside, and in their place 

 appears a prospect of a more sound, natural, and scientific basis 

 being made the foundation of our future salmon cultivation, the 

 absolute necessity of opening up the natural breeding-beds of 

 the fish will, it is hoped, become patent to every one, and the 

 dream of my old friend the late William J. Ffennel, the father, 

 so to speak, of our modern salmon fishery legislation and salmon 

 river cultivation may at last be realised. " If I live," he said 

 to me one day (I hardly care to remember how long ago it was, 

 or how soon after he was taken from us), "I shall never rest 

 until every weir and mill-dam in the three countries — England, 

 Ireland, and Scotland — has a thoroughly good and permanent 

 salmon ladder built upon it, or into it, or around it. We have 

 shown we can restore the fisheries ; we must now restore the 

 rivers. That, sir, is the true position to take up, and that must 

 be our next aim." Had Mr. Ffennel lived, river restoration 

 would probably have progressed more than it has during the last 

 decade. Mark Heron 



June 9 



[The falls referred to in our note on the Fishery Board fo r 

 Scotland last week (p. 129) are the Falls of the Tummel. — Ed. 



A RARE BRITISH HOLOTHURIAN 



OF the six species of Holothurians with shield-shaped 

 tentacles (the Aspidochirota;) that are known to 

 occur on the shores of the North Atlantic Ocean, two — 

 H. obscura and //. agglutinaia — were so shortly described 

 by Le Sueur as to be still strange to American naturalists ; 

 no definite statement as to the presence of a true, that is, 

 aspidochirote, Holothurian in the British seas has ever 

 made its way into any systematic revision or synopsis of 

 the class. 



Shortly, however, after the publication of Forbes' 

 " British Starfishes," Mr. Peach of Gorran Haven, Corn- 

 wall, published in the Annals and Magazine of Natural 

 History for 1S45 (vol. xv. p. 1 71) a short article on the 

 " Nigger" or " Cotton-Spinner " of the Cornish fishermen, 

 in which he quite rightly remarks that no typical Holo- 

 thurian with twenty tentacles had been observed by Forbes, 

 and exhibits a just pleasure in being able to say that he 

 had discovered one. Later, two Irish naturalists — Prof. 

 Kinahan and Mr. Foot — separately noted the existence 

 of what one called Cucumaria niger and the other Holo- 

 thuria niger. With an exception to be mentioned imme- 

 diately, no writer has for nearly forty years given the 

 least indication of a knowledge of the existence of this 

 " Cotton-Spinner," and it may therefore be supposed that 

 it was always with interest that I examined any form 

 that came from the British seas. A short time 

 since, on opening a Holothurian that had been in 

 the British Museum for nearly twenty years, I found 

 that, instead of those tubules which, arising from the 

 wall of the cloa;a, were first seen by Cuvier, and called 

 Cuvierian organs by Johannes Muller, being small and 

 inconspicuous, or, as often happens, altogether absent, 

 they formed rather a large, almost solid, compact mass of 



