120 



♦ KNOWLEDGE ♦ 



[Feb. 13, 1885. 



from tbe circumstance that one sweep might be maile -n-hen 

 the air was exceptiuually clear, wheu moonlight was 

 wanting, and other conditions for showing faiut stars 

 favourable (amongst other causes, difference of observing 

 power among the tix who took jiait in the work must be 

 taken into account), while the next sweep might be made 

 under conditions unfavourable for the work. This, how- 

 ever, is only a possible causa of circular striation, though 

 in so long a seiies of observations it must inevitably have 

 occurred at times, and so had ccrlainlij a share in producing 

 the peculiarity in question. But ttitre was also a sure and 

 certain cause of striation. The field of view of a tele- 

 scope is a circle, and in "sweeping" the centie of one 

 field runs along a certain arc, while the next field is 

 taken a certain distance south of that arc (or north 

 of it, according to the way the ob-erver works). Say 

 the field is half a degree in diameter, and the change 

 north or south for successive sweeps nearly as great of a 

 degree, so that one field only overlaps by a little the field 

 next norih or south of it. Then it is clear that the chance 

 of discerning a faint star near the course along which the 

 centre of the field sweeps is much "reater than the chance 

 of discerning a star where tbe fields overlap ; for in one 

 case a whole diameter of the field is available for search, in 

 the other only a short arc. In sweeping, the star will not 

 escape in one case if it be seen at any part of the compara- 

 tively long time during which that diameter is pasiiug ; 

 but in the other case, if it be not caught while the short 

 arc is passing it will not be caught at all. Thus, it is abso- 

 lutely certain that fewer stars will escape along or near the 

 tracks of the centres of the sweeping fields than midway or 

 nearly midway, between the tracks of the centres. A con- 

 centric circular striation must necessarily result. To this 

 must be added the probability that, however carefully I 

 marked in my ninety-two circles there may have been 

 slight departures from their true positions, whereby some 

 of tbe zones were made slightly wider or slightly narrower 

 than they should have been. This would make the striation 

 more marked in some places less marked in others than it 

 would otherwise have been, but, on the whole, would help 

 to make it coarser, and therefore mare obvious.* 



But, thus explained, the circular striation is of no more 

 moment than the linear striation in engravings. 



(To be continued.) 



* It is interesting to notice how inevitably any peenliarity in the 

 method of distribation in Each cases is bound to show itself. I 

 remember being very much struck by an example of this which 

 arose when I was endeavouring to secure perfectly equable chance 

 distribation for comparison with the unequal distribution of stars 

 of various orders, which I regard as so important a feature of the 

 stellar heavens. After trying various methods I thought of the 

 following : — I drew a square 10 inches in the side, and divided it 

 into 10,000 squares by equidistant parallel lines in pencil, 100 eat-h 

 way. Opening then a book of logarithms at random, I brought 

 down a pencil point at random on the tables of figures, taking out 

 the digit nearesit the point. When I had obtained four digits in 

 this way — as say 4725, I regarded the two first as showing the 

 number of divisions I was to take along one side of the square, in 

 this case -!7, and the other two as showing the number of divisions 

 I was to take along an adjacent side, in this case 25, — where these 

 divisions crossed, that is where the 47th row fx-om the left crossed 

 the 25th from the right, was the square in the middle of which I 

 was to set a point. So I went on until I had marked in many 

 thousands, and indeed tens of thousands of points. For I got others 

 to help me in the work. Here surely was a method of pure chance, 

 bound to result in equable distribution. But no. When the work 

 had gone far enough, I rubbed out the pencil Ukcs, and found a 

 marked tendency to parallelism in certain bands where there were 

 more dots than elsewhere, this parallelism showing itself on the 

 vertical zones corresponding to the numbers 2, 3, ... 5, 6, ... . 



8, 9, 10, . . . 12, 13, . . . 15, 16 18, 19, 20, . . . 22, 23, 



.... 25, 2G 28, 29, 30, and so on, and on the horizontal 



bands corresponding to the same numbers. The light bands corre- 



THE ENTOMOLOGY OF A POND. 



By E. a. Butler. 



THE MARGINS. 



ALL the insects hitherto considered spend more or less 

 of their life either actually in or on the water, and 

 are therefore correctly designated aquatic ; but there still 

 remain others, which, though terrestrial during their whole 

 career, must yet receive a passing notice, since they are 

 closely associated with water, which, indeed, seems to be 

 almost as essential to their existence as to that of their 

 truly aquatic brethren. They constitute the very abundant 

 fauna that sports about on the banks and round the margins 

 of our pands, and consist chiefly of beetles, which belong 

 for the most part to the two sections, Geodephaga, or car- 

 nivorous ground-beetles, and Brachelytra, or rove-beetles, 

 the characteristics of the latter of which we considered at 

 some length on a former occa.sion (see Kkowledge, Vol. Y., 

 p. 26). ^Suppose we have a gently- shelving bank, with plenty 

 of loose stones lying about over the partially-consolidated 

 mud of which it is composed ; when bright sunshine lights up 

 the scene this .spot will swarm with life, little beetles rush- 

 ing about, hither and hither, as fast as their legs can carry 

 them, and that is sometimes at a truly marvellous pace. 

 Bat the liveliness of the scene will be apparent only to the 

 close observer ; if any one, without gazing at any one spot 

 in particular, merely takes a general view from the usual 

 level of his eyes, he ?«r'y become dimly conscious of a few hazy 

 f. rms moving about at his feet, but no more than that ; and, 

 in all probability, he will see no living being at all, and will 

 think the place utterly barren of life, but let him condescend 

 to stoop, or better, go down on his kuees, and fix his eyes 

 on some definite spot for a few moments, and he will soon 

 see first one queer little form, and then another and another, 

 crossing the field of view, till at last, when his eye has 

 become familiarised with the outlook, and he is able to cover 

 a wider area without losing sight of the details, he dis- 

 cjvers multitudes of creatures trotting about in all direc- 

 tions, each intent on the absorbing business of its own little 

 life, and he wonders how it was possible for him to have 

 missed seeing them before. If the isame spot be visited on 

 a dull day, the population will appear to be much more 

 scanty ; many of the inhabitants will have retired under 

 shelter, and must be 'sought for under the stones, etc. If 

 there be simply muddy banks, or better still, a broad mud 

 flat, only recently left liare by the lowering of the level of 

 the water through evaporation, vast numbers of tiny rove- 

 beetles will be seen running about in all directions, many 

 of them turning up their tails over their backs, as though 

 they would utilise them as umbrellas or sunshades. If, 

 again, there be here and there jiiles of dead leaves, bits of 

 sticks, or other rubbish, washed up and stranded when the 

 water stood at a higher level, these heaps, on being turned 

 over or shaken above a piece of paper, will often yield 

 abundance of beetles of various kinds, which have been 

 putting up there in temporary lodgings, or perchance have 

 been finding therein excellent board as well as lodsins. 



S"me of these riparian beetles are adorned with most 

 brilliant metallic colours — green, golden, bronze, coppery; 

 and, as they dart swiftly about in the sunshine, like flashes 

 of light, they look marvellously beautiful. One of the 

 handsomest, Ancliomemis marginatus (Fig. 1), is a fairly 



sponded to the vertical and hoi-izontal bands numbered 1, . . . 

 4, .... 7 11, ... 14, .... 17, .. &c. The explana- 

 tion was simple enough. The figures representing 1, 4, 7, cover 

 less space than the others, and by my method of taking out 

 digits there was more chance of taking one of the fat figures, 

 0, 2, 5, G, 8, and 9, than one of these lean figures, 1, 4, and 7. 



