86 



NATURE 



[March 2;, 1913 



Protodrilus appears to inhabit only the European seas, 1 

 having been taken in the Black Sea, the Mediter- 

 ranean, at Heligoland in the North Sea, and at Amble- 

 teuse, on the French side of the Straits of Dover. 

 Protodrilus was found on March 2 in a small bay just 

 outside and to the east of Plymouth Sound. On 

 March 11 the spot was again visited, and a large 

 number of specimens, more than a hundred, gathered 

 in about an hour. The animals were found almost 

 at the high-water mark among stones and gravel at a 

 point where a small stream of fresh water runs into 

 the sea. 



It is an interesting fact that the animals are 

 immersed at one period in practically fresh water, and 

 at another period in sea water; samples of the water 

 in which the animals were living taken at low water 

 during the neap and spring tides were found to have 

 densities as indicated by a hydrometer of about i-ooi 

 and i-ooq respectively; while the density of a sample of 

 sea water taken just outside the breakwater at Ply- 

 mouth, estimated by the same instrument, was found 

 to be about 1-025. - 



These specimens of Protodrilus are undoubtedly 

 different from those previously taken at Ambleteuse 

 and Heligoland, but they resemble in some characters 

 both the Mediterranean' forms, Protodrilus flavocapi- 

 tdtus, Uljanin, and Protodrilus spongioides, Pieran- 

 toni. The former of these species occurs in situations 

 which are never covered by more than a few decimetres 

 of water, while the latter is represented by only four 

 specimens taken from fresh water. A fuller investiga- 

 tion of the English specimens is being undertaken in 

 order to compare them in more detail with the known 

 species of this genus. 



The English Protodrilus were living in the situation 

 described above along with Gammarus marinus, 

 an Oligochaete and Gunda (Procerodes) idvae; the 

 latter of these species was first taken in this spot in 

 great numbers two years ago, and does not appear 

 to have been recorded previously on the English coast. 



J. H. Orton. 



The Laboratory, Citadel Hill, Plymouth. 



was really invisible while the telescope was fixed, but 

 by imparting to it a slow oscillation right and left I 

 kept the signal in view w : ith sufficient distinctness to 

 see the ball drop, although I was not certain it had 

 really dropped until a second or so afterwards, owing 

 to the great faintness of the image observed. 



I recollected then that, often, in similar conditions 

 of seeing, having picked the signal without any diffi- 

 culty while "sweeping" for it. I had failed to see it 

 afterwards, and gave up the attempt, thinking I had 

 been mistaken, or that the mist had become thicker. 

 I have therefore no doubt as to this most curious and 

 inexplicable fact : an indistinct object is better 

 seen in a slowly moving telescope than in 

 the same telescope when kept steady. There 

 must be a very interesting physiological property of 

 the eye involved in producing this result, which is 

 quite in opposition with what one would naturally 

 expect. Perhaps some of your readers have noticed 

 something similar, and could throw a little light on 

 this mysterious phenomenon. 



M. E. J. Gheury. 



Woolwich Polytechnic, March 15. 



On the Gain of Definition obtained by Moving a 

 Telescope. 



The following is an account of a very singular fact 

 which came recently under my notice, and for the 

 explanation of which I am absolutely at a loss. 



I am in the habit of rating my chronometer by 

 means of the time-ball dropped at the Greenwich 

 Royal Observatory, about 3! miles away, a signal 

 which I observe in a small hand telescope. 



On March 11, the weather being misty, I failed to 

 pick the signal post, although I knew exactly where 

 |it was, and had placed the telescope exactly in the 

 right direction. I moved the telescope - a little, think- 

 ing I had displaced it in putting my eye to the eye- 

 piece, and I immediately saw, very dimly, the dome 

 of the observatory, and the signal, with the ball at 

 half-mast, and noticed that they were in the centre 

 of the field all the time. As soon as I steadied the 

 telescope, however, they vanished completely. They 

 reappeared as soon as I began to " sweep " for them, 

 but remained discernible only while the motion lasted. . 

 I repeated the experiment several times ; the signal 



nd Flora des Golfes 



Neapel.*' "Vol. 

 mperatures betwe 



1 X". Pierantoni, " Fauns 

 Protodrilus. 1-008. 



- These values of density were made at temperatures between 15 and 17 

 C, and are to he regarded as approximations only to the'absolute density : 

 :;^ the water in which the Protodrilus were living' would be constantly 

 changing, it, was not considered worth while to analyse accurately two 

 'ample 



Four-horned Sheep. 



Mr. Ritchie's note on four-horned sheep in Nature 

 I of March 6 is interesting, but I am inclined to doubt 

 whether there ever was, in Scotland or any other 

 country, a breed in which four horns are normal. 

 No doubt it is possible to fix this character in the 

 male sex by careful selection, as has been done by 

 some breeders of the spotted or Barbary sheep (some- 

 times called Spanish, Syrian, or Zulu sheep); but even 

 these have not succeeded in fixing the character in the 

 female sex. I have evidence, in the shape of specimens 

 or photographs, of the existence of four-horned sheep 

 in North and South Africa, Mongolia, China, the 

 Himalayas, Baluchistan, and Chile. The Iceland 

 breed was supposed to be four-horned, and no doubt 

 four-horned examples were often found amongst them, 

 a specimen I have being precisely similar in type to 

 an abnormally four-horned Shetland. 



Mv own experience of four-horned rams is that in 

 most cases the lower horns, and in some cases the 

 uoper also, require to be cut at some time in their 

 life to prevent them from growing into the cheek, 

 or below the jaw, so that the animal cannot graze ; 

 and this no doubt would have a tendency to eliminate 

 the four-horned rams where not specially selected. 

 No instance is on record, so far as I know, of any 

 wild sheep having more than two horns, neither have 

 I seen any skull of domestic sheep in which there 

 were more than four horn-cores, though five-, six-, 

 and even eight-horned sheep have been recorded. 



H. J. El wes. 



Colesborne Park, near Cheltenham, March 14. 



THE EXPERIMENTAL STUDY OF FLUID 

 MOTION. 1 



MANY attempts have been made to study the 

 motion of fluids past an obstacle by ex- 

 perimental methods, and experiments made for 

 this purpose may be divided roughly into two 

 classes : 



(a) Those in which the fluid is made to flow 



1. The figures which accompany this article are from the Technical Report 

 of the Advisory Committee for Aeronautics for the year 1011-12, and are 

 reproduced with the permission of the Controller of H.M. Stationery Office. 



NO. 2265, VOL. 91] 



