November 1, 1897.] 



KNOWLEDGE. 



263 



been Biela'a comet. The last certain observation of it 

 took place in the autumn of 1852. The comet would 

 now appear to be represented by a meteor stream which 

 we encounter towards the end of November, A fine 

 display was noted in November, 1872, and again thirteen 

 years later in November, 1885. We may therefore look 

 forward confidently to a striking display in 1898. It 

 should be noted that though the mass of the meteors 

 giving rise to the shower move in the same orbit as the 

 comet, they are not coincident with it, but in the case of 

 the shower of November, 1872, were following the com- 

 puted place of the comet at a distance of about twelve 

 weeks. 



It may be interesting to recall that the comet discovered 

 by Mr. Perring in the end of last year (Comet G, 1896), on 

 1896, December 8, had an orbit very closely resembling 

 that of Biela in every respect except the position of its 

 major axis. — E. Walter Maunder.] 



THE MfiTRE AND THE INCH. 

 To the Editors of Knowledge. 



SiKs, — I am not aware that the following argument for 

 keeping the metre and the inch in concurrent use has been 

 published hitherto. 



Suppose an artificer has to make a vase true to a scale 

 pattern drawing. He wishes to test his work. If on 

 measuring his drawing across at any height from the base 

 with a straightedge divided into eighths of an inch he 

 finds, say, 10 eighths, and again measuring the circum- 

 ference of his work at the corresponding height with a 

 centimetre tape he finds it 16 centimetres, then he may 

 know that his work is correct. 



This depends on the following approximate proposi- 

 tions : — 



250,000,0(10 inches = the semi-axis of rotation of the 

 earth nearly. 



10,000,000 metres = the quadrant of Paris more nearly. 

 Hence — 



2 metres : 25 inches') 

 1 metre : 12-5 inches >- = r : 1 



1 centimetre ; J inch ) 

 True to about 1 in 400. T. Wilsos. 



Harpenden. 



♦ 



Scifn« Notts. 



While gold is justly considered as one of the rarest 

 metals, it is also one of the most widely diifused, as is 

 shown in the September Number of Popular Sci,.-nri\ in a 

 description of an assay of clay within the limits of the city 

 of Philadelphia. A block of clay a cubic foot in volume, 

 as it lies in the ground, weighs one hundred and twenty 

 pounds, and the assay gives seven-tenths of a grain of 

 gold to the cubic foot, the gold being very uniformly 

 distributed. Under the streets and houses lie four thousand 

 one hundred and eighty million cubic feet of clay, in which 

 securely lies one hundred and twenty-six million dollars. 

 And if, as is pretty certain, the corporate limits of the 

 city would afford eight times this bulk of clay, we have 

 more gold than has yet been brought, according to statistics, 

 from California and Australia. 



A meteoric stone, weighing eighty tons, the largest in 

 the world, was found some years ago in Greenland by 

 Lieutenant Peary. It was transported this autumn to 

 Sydney, Cape Breton Island, on board the steamer Hope, 

 and is to be landed at New York. 



ENGLISH PLAINS AND ESCARPMENTS. 



By E. Ltdekkek, B.A.Cantab., F.R.S. 



HAYING lately been conducting a small party of 

 military students on a short geological tour in 

 the West of England, in the course of it I have 

 been much struck with the remarkable similarity 

 of the scenery of diiJerent districts, due to simi- 

 larity in the mineralogical identity of geological formations 

 of widely different ages. This is, of course, a fact well 

 known to geologists, and one with which I have long been 

 personally familiar, both in this country and in India. 

 Nevertheless, the phenomenon impressed itself very deeply 

 in my mind on this occasion ; and as it is one with which 

 many of the readers of Kxowxedge may be unacquainted, 

 a brief account will probably be found interesting, in spite 

 of the fact that I have nothing new to communicate. 



If a resident in the neighbourhood of Chatham or 

 Rochester take a walk to the summit of the North Downs at 

 Bluebell HUl, he will come upon what is technically known 

 as the escarpment of the Chalk — that is to say, upon a line 

 of irregular and somewhat abrupt cliffs in which the strata 

 are sloping, or dipping, from the face of the escarpment 

 towards the centre of the hill. Below him lie some irregular 

 steps or hillocks of lower Chalk and marl in advance of the 

 main escarpment, and then he sees, stretching out for a 

 distance of about a mile in transverse extent, a flat plain 

 formed by the blue clay of the Gault, through which the river 

 lazily wanders, and whose surface is thickly dotted with oak 

 trees. This (rault formation immediately underhes the 



' Chalk and its associated sands and marls, and is known to 

 geologists as the Gault plain ; its northern limits being 



j frequently defined by a line of springs arising from the 

 supersaturated Chalk. 



Let him next descend the Chalk escarpment and walk 

 across the Gault plain in the du-ection of Maidstone, when 

 he will come upon another range of hills considerably 

 lower than the Chalk, and consisting partly of pure white 



\ and yellow sands and partly of impure limestones (Kentish 

 Eag). These hills are constituted by the Lower Greensand 

 formation, which underlies the Gault, and continues as far 

 as the vUlage of Yalding, situated on the Medway some 

 distance above Maidstone. Here he comes upon another 

 line of escarpment running in an easterly and westerly 

 direction ; and below him lies another plain identical in 

 appearance with the Gault plain he has left behind, only 

 of considerably wider extent. There is the same black 

 heavy soil, forming an unbroken flat intersected by the 

 same stream, bearmg a similar crop of oak forest, and 

 defined by a similar line of springs on its northern boundary. 

 Looking on this tract, anyone not a practical geologist 

 would almost certainly say he was on the same stratum as 

 the Gault plain. Nevertheless, the two plains are separated 

 by the whole thickness of the Lower Greensand ; and 

 whereas the Gault clay is of marine origin, the Weald 

 clay, which is the formation exposed at Yalding, was 

 deposited in fresh-water, as is proved by the nature of its 

 organic remains. 



If the traveller next passes to the districts lying to the 

 north-west of the Chiltern Hills, or into Dorsetshire, and 

 thus reaches the country of the Oolites, he will find several 

 flat tracts of clay which might also quite easily be mistaken 

 for the Gault plain. For instance, in the neighbourhood of 

 Swindon the Portland Oolite, which is one of the highest 

 beds of the .Jurassic or Oolitic series, escarps on to the 

 plain of the Kimeridge Clay ; while the Coral Rag, which 

 underlies the latter, forms a second escarpment on to the 

 still older Oxford Clay, as is well seen in the neighbour 



