

able valve ; heavy weights are then fastened to the foot to keep 

 !.'!.- mail at tlio bottom. Sounds made uudur water are oonduoted 

 : able distance, and honoe by tap* on the aide* 

 tall message* are tra surface. One of 



those plans is frequently used for the recovery of proper: 

 unkon vessels, and for fixing tackle, so as to endeavour to raise 

 !!. surface, and largo amounts of treasure have fro- 

 . been thus roc<.. M- action of the 



pump, for forcing down tho air, will bo explained in u future 

 HMOB. 



air is a material substance, it has weight, and wo mnst 

 how to prove tliis, and uUo to ascertain what its weight 

 is. 



. i-h mi ordinary substance, we have merely to place it 

 in one scale of a balance, and place our weights in tb 

 This plan, however, will not answer hero, since the air presses 

 on both ; \\>- have, therefore, to proceed in a different way. A 

 large glass globe, having an opening which can bo closed by a 

 k, is procured, and by means of an air-pump it is com- 

 :.-d of air, and very accurately weighed ; the air is 

 .iitti-ii to it, and tho difference thus produced in tho 

 t-ight accurat.-Iy noted. By now filling tho globo with water 

 r. intents may easily bo ascertained, and from this wo 

 u calculate the weight of a cubic foot or any other volume of 

 air. 



In this way it is found that 100 cubic inches of air weigh, 



rdinary temperature and pressure of the air, a little over 



us, and hence a cubic foot weighs about l oz. Though 



this weight appears small, and actually is HO, when compared 



with the weight of solids or liquids, yet if wo calcnl: 



of air in any building we shall find it much more than 

 we expected. Suppose, for instance, we have a room 20 feet 03' 

 15 and 10 feet high, it contains 3,000 cubic feet. The air in it 

 therefore weighs about 3,750 oz., or rather more than 2 cwt. 

 The weight of other gases may be ascertained in a similar way, 

 and by comparing tho weights of equal bulks of them with that 

 of air, at tho same temperature and pressure, wo can ascertain 

 their specific gravities. 



In making this experiment an air-pump was required ; and as 

 this piece of apparatus is necessary in nearly 

 all pneumatic experiments, it will be as well 

 to explain its construction at once, before 

 passing on to notice the effects produced by 

 the weight of the air. 



The simplest instrument for removing tho 

 air from any vessel is that known as the 

 exhausting syringe, and is represented in 

 Fig. 1. A is the globo from which the 

 air is to be removed ; this is furnished 

 with a stop-cock, B, and screws on to 

 the end of tho syringe ; c D is the cylinder, 

 which is accurately turned inside, and in 

 which the piston E works air-tight. In order 

 to prevent leakage past the sides of this, a 

 groove is turned in it, as shown, and cotton 

 or some similar packing is wound tightly 

 round, and then saturated with oil. In this 

 way it is made to fit much more tightly, and 

 the wear is likewise greatly diminished. A 

 pipe, closed by a stop-cock F, opens into the cylinder near its 

 lower end. 



Let the piston be at tho lower end of the cylinder ; the valve 

 F must now be closed and B opened ; the piston is then raised 

 to the top, and the air contained in A will expand and fill tho 

 cylinder. B is now closed, and p opened, and as the piston 

 descends it will force the air contained in the cylinder through 

 F. The taps are again reversed, and a similar process repeated 

 till nearly all the air is removed. 



If the area of the cylinder be just equal to that of the globe, 

 one-half the air in tho latter will be removed by the first stroke, 

 and tho density of that within wi :1 be -.V of what it was ; similarly, 

 after the second stroke it wi,. r tho third, fa, and so on. 



It would be found very inconvenient, however, to open and 

 close the taps continually ; in practice, therefore, they are re- 

 placed by valves, which are constructed of a small piece of oiled 

 silk covering a hole bored in a metal plate. Those only allow 

 of the passage of the air in one direction, and that at F ia made 

 to open outwards, while B opens upwards. 



-:. will awwer wU f r ***** a glob*, bet the 

 .out!, of thu an** neoMstrily be mail, and K, but few tku>r 

 could t* introduced od operated upon. The pip* (ran tiu, 

 bottom of the syringe is therefor* prolonged, and ta*de to t*~ 

 through the outre of a fixed taeUl di*e, the enrfnoe of wU* fr 

 ground so as to be perfectly true. Olase reed rets are else 

 ground true on their open ends, so that whe0 a UttJo tallow u 

 smeared on to fill the small irregularities of the Tf-rrr mo air 

 can pass between them and *e pump-plat*. Any ( 

 can then be placed 00 the plate, and have the air 

 from it. 



LESSONS IN GEOLOGY. XX. 



THE PEBMIAX BTBTEM. 



ON each side of the carboniferous system there is an 

 blage of rooks in which sandstones predominate; 

 iron which occurred in such quantities in the carboniferous period 

 as to form the bands from which we derived our <*M supplies of 

 tho metal is four -J in each of these systems in the Ute of an 

 oxide, tinting tho sandstones red. Hence they hare been nemH 

 tho Old Bed Sandstones and the New Bod Sandstones respectively. 

 Formerly tho term ]>oikilitic was applied to the strata which is 

 enclosed between the lias and the carboniferous. The word is 

 derived from woutlKot (poi'-ki-los), variegated, because the red 

 rocks are frequently streaked and mottled with lighter colour, 

 or with green and bluish tints. 



But on a closer examination it was found that this group 

 would bear a division. The fossil* contained in the lower 

 half exhibit close affinities to those of the carboniferous period, 

 while those of tho upper division hare an equally close relation 

 to tho next groat system the oolite. Hence the line which 

 divides the palaeozoic and the meeozoio periods runs in the 

 middle of tho old poikilitic group, and divides it into two recog- 

 nised systems the Permian and the Trias. 



The namo Permian is derived from Perm, a Bussian govern- 

 ment, and was bestowed upon the system by Sir Boderick Mar- 

 chison in 1841, because in that locality the rocks of the system 

 are widely and typically developed. Professor Ki&g has thus 

 arranged the members of the system, with their German equiva- 

 lents : 



NORTH OF ENGLAND. 



1. Crystalline, or concretionary and nou- crystalline limestone. 



2. Brecciated and pseudo-brecdated limestone. 



3. Fossiliforous limestone. 



4. Compact limestone. 



5. Marl-slate. 



6. Inferior sandstones of various colours. 



THtTRINGlA. 



1. Stinkstein. 



2. Kauchwaeko. 



3. Dolomite, or Upper Zechstein. 



4. Zechstein. or Lower Zechstein. 



5. Mergcl-schiefer, or Kupferchiefer. 



6. Bothliegendes. 



It will be observed that the above series may be divided gene- 

 rally into limestones and sandstones, both of which are peculiar. 

 Adhering 1 to our usual course, we will take the lowest first. 



The Sandiitoiies attain a considerable thickness 200 feet ia 

 Durham. They are red, though occasionally white quartoce, and 

 frequently pebbly, or conglomerate. Their German representa- 

 tives are termed Rothlic-jcndes, or red lien, being so called by 

 the miners who work tho slaty marlstono which lie* upon them 

 for copper pyrites, with which it is richly impregnated. When 

 the red sandstones are reached, they know that the copper de- 

 posit has ended ; hence they are called the red under-lien. 



As we pass up from the sandstones to the limestones we find 

 between the two marl-slates, which are not marls because they 

 contain any quantity of lime, but from mottled and friable con- 

 stitution. 



The Limestones are remarkable for the fact that they contain 

 as much as 44 per cent, of carbonate of magnesia mixed with 

 tho carbonate of lime. This is not invariably the ease, for 

 frequently the lime greatly predominates, and often the nek 

 consists of concretionary masses radiating from the centre, and 

 about the size of cannon-balls. 



Dolomite u the mineralogioal name given to this 



