Aug. 29, 1884.] 



KNOWLEDGE 



171 



Our other British -water-scorpion, Nepa cinerea (Fig. 1), 

 occi'is commonly in ponds. It is a smaller and much 

 broader insect, extremely fl'ittened (like 

 ^^ I ^ many other bugs, notably our disgusting 

 ^)Of household pest), with shorter and stouter 



/^'!liL>^ legs, but still similar in plan to Ranatra, 

 |w7\ \ which it resembles also in its brilliant scarlet 

 body. It is a most sluggish insect, and can 

 easily be secured by the hand when seen 

 near the edge of the pond ; but the diffi- 

 culty is to catch sight of it, for so exactly 

 similar is its colour to that of the mud 

 amongst which it lies, that unless the ob- 

 server can bring to bear upon it a pair of 

 keen and well-trained eyes, it will in nine 

 cases out of ten remain undetected, if only it have 

 the sense not to betray its presence by moving. In 

 consequence of its sluggish habits and mud-loving pro- 

 pensities, it sometimes becomes covered with an incrusta- 

 tion which does not render its detection any the easier. Its 

 protective coloration, no doubt, gives it chances of many 

 more meals than it would otherwise get, for unwary insects 

 will often approach near enough to come within range of 

 the hooked fore-feet without being conscious of the risk 

 they run, and are only aroused from their fancied security 

 by finding themselves suddenly clutched and pressed in a 

 deadly embrace against the sharp beak which is ever in 

 waiting to tap any juicy body that may be presented to it. 

 The breathing apparatus is a marvel of complexity ; 

 there are both spiracles and tail filaments, the latter lead- 

 ing into the two longitudinal trachea?, which run parallel to 

 the sides. From these, innumerable minute tubes ramify 

 all over the body, and if only it were possible to dissect 

 out the whole tracheal system, and separate it from the 

 rest of the body, it would form a most elegant object, and 

 appear like an exquisite network of silver filagree, built up, 

 as it were, upon a gridiron-like framework consisting of 

 two long curved side-pieces connected by arched cross- 

 pieces. In the thorax there are a few dilatations of the 

 tracheal system, in the form of air-bags, such as in many 

 other insects, especially those that are vigorous in flight, 

 may be found in other parts of the body as well. 



The eggs of Nepa are very peculiar. They are oval, 

 with seven long filaments at one end ; while they are being 

 laid, the filaments of each serve as a sort of cup to keep 

 its successor in position, but when the egg is once de- 

 posited, the filaments bend backwards and form a circlet of 

 recurved hooks. The eggs of Eanatra are more elongate, 

 and furnished with only two bristles. 



As these water-scorpions are bugs, they do not undergo 

 much change of form in the course of their life ; the larvse 

 are very similar to the adult, the chief differences being the 

 smaller size and the absence of wings and tail-filaments, 

 the place of the latter being taken by a small pointed 

 projection. 



(To he continued.) 



THE PHYSICS OF THE EARTH'S 

 CRUST. 



By Richaed A. Peoctok. 



PROFESSOR DAVIDSON, of the United States 

 Geodetical Survey in California, has noticed most 

 remarkable deviations in the direction of the action of 

 gravity in the region which he has surveyed. Deflections 

 of ten or eleven seconds of arc — which would corresjiond. 



in the determination of the position of a place by astro- 

 nomical methods, to errors of 1,000 or 1,100 feet — are 

 common. This shows that the density of the materials 

 beneath the visible surface of the earth is very irregular. 

 But what is remarkable is that the deviations of the 

 plumb-line (so, for convenience, to describe a peculiarity 

 which in reality is far too delicate to be dealt with by a 

 plumb-line observation) are not, as might be expected, 

 towards the great mountain ranges, but towards the regions 

 of depression. It would seem that the depressions indicate 

 the downward tendency of very dense matter, and a 

 resulting closeness of packing, so to speak, which makes 

 those depressed regions exert a very powerful local attrac- 

 tion, causing the plumb-line in their neighbourhood to 

 deviate towards them. On the contrary, in the neighbour- 

 hood of the mountain ranges, and even on their flanks, 

 the deflection of the plumb-line is from the regions of 

 elevation, as though vacant spaces or matter of relatively 

 small density existed beneath the upheaved portions of 

 the crust. 



Another point, still more remarkable, and apparently 

 established on sufficient evidence, is that in certain regions 

 the direction of gravity seems to have changed largely 

 during a period of less than thirty years. Professor 

 Davidson mentions one place where the position, as deter- 

 mined by astronomical methods depending on the direction 

 of gravity, has varied no less than 16 seconds of arc since 

 185-1. The arrangement of the masses beneath the sur- 

 face in the neighbourhood must have greatly changed — 

 marvellously, in fact, when the shortness of the time is 

 considered. One cannot wonder that California should be 

 a region of great earthquakes, insomuch that no large 

 building can safely be made of stone in that part of the 

 earth. 



Prof. Davidson's observations tend to throw some doubt 

 on all such methods of determining the earth's density as 

 depend on variations in the force and direction of gravity 

 in the neighbourhood of mountain masses, at the bottom 

 of deep mines, and so forth. It is certain that had such 

 observations been first made in such a region as California, 

 they would have led to entirely erroneous results, or, 

 rather, they would have failed utterly ; for it appears that 

 instead of the plumb-line being deflected towards the 

 upraised masses (supposed to be of known density), as in 

 the case of the Schehallion experiment, it would have been 

 drawn from them towards masses of compressed matter 

 beneath the lower levels, and before unsuspected. As for 

 pendulum experiments in mines, such as the celebrated 

 Harton Colliery observations, they seem utterly discredited 

 by such observations as Professor Davidson's. But, to say 

 the truth, they have long been regarded as worthy cf little 

 trust. In dealing with Airy's observations (really con- 

 ducted by his assistant, Mr. Dunkin), in Rodwell's " Physi- 

 cal Cyclopa?dia," I pointed out fifteen years ago that the 

 result obtained was altogether unreliable, chiefly because 

 of the uncertainty necessarily existing in regard to the 

 density of the regions surrounding the scene of operations. 

 The result obtained at Schehallion by Maskelyne was more 

 nearly correct, judged by the indications of the Cavendish 

 experiment, the only method which seems really trust- 

 worthy. But what was learnt from the Schehallion 

 experiments was simply that the rock masses in that region 

 are tolerably uniform in structure, and of about the mean 

 density assigned by Maskelyne, not that the earth has the 

 mean density deduced by that observer from his observa- 

 tions there. This last we learn from Baily's experiments 

 by the Cavendish method, in which the earth is weighed 

 against metallic masses of known density. 



Other observations in California show hov,- little reliance 



