o, the condenser, filled with cold water, in which the steam is condensed after it has acted on the piston.

P, the metallic piston, which moves in the cast-iron cylinder, receives directly the pressure of the steam, and transmits motion to all parts of the engine.

Q, the air reservoir of the forcing feed-pump, which conveys water into the generator.

B, the reservoir or cistern of cold water, drawn from the coldwater-pump.

s, the hot-water-pipe, which carries into the generator the water supplied by the feed-pump.

T, the cold-water-pipe, which conveys into the condenser the water supplied by the cold-water-pump.

v, the eduction-pipe, which admits the steam from the cylinder into the condenser, after it has acted on the piston.

produce the alternate motion of the piston and piston-rod, is represented in fig. 205.

The steam-pipe c, brings the steam from the generator into the cast-iron rectangular valve-box fixed on the cylinder, in which are made three ports or orifices, u, n and a. By means of an interior passage, the first u, communicates with the upper part of the cylinder; the second n, with the lower part; and the third a, with the eduction-port r, leading to the condenser. Over these three orifices or ports a piece t, called the slidevalve, passes smoothly in succession. This piece is fixed to a rod b, jointed at m to a larger rod d, and it receives along with it the backwards and forwards motion of a bent lever yos, communicated to it by the eccentric. When the slide-valve is at the top of its course, as shown in the figure, the steam enters by the port n, and proceeds to the lower part of the cylinder, while the port u, being covered by the slide, the steam cannot enter it; but that which is above the piston passes by the same port u, and by the port a, into the eduction-port, whence it proceeds to the condenser. The piston is, therefore, only acted on by an upward pressure, and ascends. If, on the contrary, the slide-valve be at the bottom of its course, the steam enters by the port u, and the port n admits a free passage to the steam below the piston into the condenser; consequently, the piston descends; and the same process is repeated at every upward and downward movement of the slide-valve. As to this movement, it is communicated by the eccentric. This name is given to the circular piece E fixed on the horizontal b, the slide-valve-rod, communicating motion to the slide-shaft A, but in such a manner that its centre does not coincide

v, the fly-wheel, made of cast-iron, which is fixed to and revolves with the horizontal shaft, and is employed to regulate its motion, in consequence of its inertia, especially when the piston is at the top or the bottom of its course.

Y, the bent lever, which transmits the motion from the eccentrice to the slide-valve rod b.

z, the eccentric-rod, which transmits the motion from the eccentric to the bent lever.

a, the eduction-port, or orifice, which communicates alternately with the upper and lower parts of the cylinder, and gives a passage to the steam through the eduction-pipe u into the condenser.

piston.

valve, which admits the steam alternately above and below the the steam-passage, by which the steam from the generator is admitted into the valve-box.

d, the stuffing-box, in which the piston-rod slides, and which is made steam-tight.

the eccentric, fixed on the horizontal shaft, in a hoop or collar, to which the eccentric-rod is attached. m, the link which connects the slide-valve-rod & with the bent lever v and the eccentric.

In the figure just described, the lower parts do not represent exactly the arrangement of the pumps, and the reservoirs or cisterns of hot and cold water, adopted in actual practice. The modifications here given to them have been made with the view of enabling the student to comprehend more readily how the different parts act and communicate with each other. Valve-box and Eccentric.-The apparatus for the distribution of the steam to the different parts of the cylinder, in order to

with that of this shaft. The eccentric is surrounded by a
this hoop are fastened the links or bars z z. The hoop, with
hoop or collar c, in which it is made easily to revolve. To
horizontal direction, and thus an alternate motion is communi-
out revolving, receives the motion of the eccentric, in the
described.
cated to the lever soy, and from it to the slide-valve, as above

the steam acts only on the upper side of the piston, are called
Watt's Single-Acting Engine.-Those steam-engines in which
to the other end of the working beam, the piston of the single-
Single-Acting Engines. By means of a counter-weight attached
acting engine is made to ascend to the top of the cylinder.
These engines, which are but little used now a-days, were at
first chiefly employed in the draining of mines. For this pur-
pose they are preferable, on account of their simplicity of con-
struction, to the double-acting engines; and they are still used
with great effect in the mining districts of this country,
especially in Cornwall, A section of a single-acting engine is

represented in fig. 206. The working beam, B B, is made of sphere of metal, fig. 207, which turns freely on two pivots. wood; and at its extremities are constructed arcs of a circle On the extremities of a diameter are fixed two short tubes on which two chains are fastened; to one of these chains is open laterally, and in contrary directions, the orifices being attached the top of the piston-rod; and to the other, the top intended for the escape of the steam. In order to introduce of the pump-rod; r being the piston, and a the counter-water into this sphere, it is first heated so as to rarefy the air weight. On the right of the cylinder a, is placed the valve- it contains, and then immersed in cold water; thus the air box, into which the steam is admitted from the generator by contracts and the water enters the sphere by the orifices. If, the pipe T. A rod d, carries three valves, m, n, and o. The then, this vessel be heated to the boiling point, the steam as it valves m and n open upward, and the valve o opens downward. issues from the orifices will communicate to it a rapid motion When the valves m and n are open, as shown in the figure, the of rotation, which arises from the pressure of the steam on the steam is admitted by the pipe r with its full pressure, to act sides of the tubes opposite to the orifices. Several attempts on the piston P; whilst that which is below is admitted into have been made with the view of applying to practical purthe condenser N by the eduction-pipe м, and the piston poses, and on a great scale, the reaction of the steam in such descends. Now, the rod which carries the valves m, n, and o, an apparatus, as a moving power. Experiments have also been is attached to a bent lever de k, which moves on a hinge c, made to make the steam act by impulsion, by directing the and opens and shuts the valves. In order to effect this, a rod force of a jet of steam on the vanes of a revolving wheel. But F, which is fixed to the working beam, carries two knobs or in these various processes, the steam has always failed to proprojections a and b, by means of which it presses on the ex- duce any useful effect which could be at all compared with its tremity k of the bent lever. In the figure, it will be seen that, action when directed against a piston. according to the arrangement of the valves, the piston de- High, Low, and Mean Pressure Engines.-Besides the distincscends, and with it the rod r; consequently, the knob b strikes tion of steam-engines into the two classes of Double and Singlethe lever and makes it descend along with the rod d mo; the Acting Engines, they are also divided into high, low, and valves m and o will then close, and the valve n open. At this mean pressure engines. An engine is said to be a low-pressure instant, all communication with the generator and the con- engine, when the tension or elasticity of its steam does not denser is interrupted; but the steam which causes the piston exceed that of 14 atmospheres; that is, the pressure of 37 to descend passes freely below it by the passage c. Pressing, inches of mercury, or of 181 lbs. on the square inch, which is then, equally on the two sides of the piston, it balances itself, 3 lbs. above the ordinary pressure of the atmosphere. An and in consequence of the traction exerted by the counter-engine is said to be a mean-pressure engine, when the elasticity

weight o, the piston ascends; this requires little force, because of the steam is comprised within the limits of that of 14 atmos the mining-pump, the rod of which is fixed to the weight a,pheres and that of 4 atmospheres; that is, when the pressure only requires assistance when its piston ascends. moment when the piston r reaches the top of its course, the mercury, or of 59 lbs. On the square inch, which is 441 lbs. above At the lies between the preceding pressure and that of 120 inches of knob a strikes in its turn the lever k, raises the rod dno, and the ordinary pressure of the atmosphere. An engine is said to the steam is admitted on the piston, which begins to descend; be a high-pressure engine when the elasticity of the steam ex and so on, alternately. ceeds that of 4 atmospheres. The preceding distinction of engines into their several kinds, according to the pressure of their steam, is that which prevails on the continent. with its full force on the piston during the whole stroke of Expansive and Non-expansive Engines.-If the steam presses the engine, that is, during the whole course of the piston froly one end of the cylinder to the other, its elastic force is nestry the same throughout, and we say, that the steam does not work

Reaction Engines.-Machines in which the steam acts in the same manner as the water in the Hydraulic Tourniqy t, p. 108, vol. iv., are called Machines or Engines of Reaction. The idea of these machines is very ancient; Hero of alexandria, 120 B.C., the inventor of the fountain which still bears his name, has described the following apparatus known under the name of the Acolipile of Reaction. It consists of a hollow

expansively; but if, by a proper arrangement of the valves, the power, considering the difference to be due to their loss of steam is cut off when the piston is at or of the stroke, then power by friction and other causes. In the manufactory of it works expansively; that is, in consequenes of its expansive Messrs. Boulton and Watt, at Soho, this dynamical unit was force, the steam still continues to act on the piston, and then ultimately reduced to 32,000 pounds raised one foot high per completes the stroke. Hence arises the distinction between minute. If we reduce this dynamical unit to the proposed those engines called Expansive Engines, and those called Non-one, we shall find that a horse-power, according to Watt, expansive Engines. There is also another classification of was equal to 550 dynams; or, according to his later estimate, to 533 dynams.

Fig. 207.

Among the engineers in France, the mechanical power of steam-engines is measured by a dynamical unit called chevalvapeur (that is, steam-horse); and this unit represents the force required to raise 75 kilogrammes, one metre high, in one second; that is, 75 times the French dynam, which i called the kilogrammetre. According to our previous calculations, the cheval vapeur, or French horse-power, is 75 X 7·238 = 542.85 dynams per second; or 32,571 pounds, raised one foot high per minute. This dynamical unit being nearly a mean between Watt's two separate estimates of a horse-power, it is quite evident that the French engineers have adopted it so as to be as nearly as possible in accordance with our estimate of a horse-power.

Locomotives.Steam-engines, called locomotive engines, or simply locomotive, are those which, mounted on a railway carriage, move from place to place by transmitting their motion to the wheels. In these engines, the working-beam, the parallel motion, and the fly-wheel, necessary to fixed engines, are dispensed with. The form of the generator or boiler is also completely modified. The principal parts of these machines are the frame, the fire-box, the cylindric generator, the smoke-box, the steam-cylinders with their valves, the drivingwheels, and the apparatus for feeding the generator.

The frame, which is made of oak, rests on the axles of the engines into Condensing Engines and Non-condensing Engines. wheels, and supports all the parts of the engine, which is The condensing engines, as the name implies, are those which represented in fig. 208. are furnished with a condenser where the steam is liquified or the engine-man or conductor, who directs the locomotive; he This figure shows the position of condensed after it has acted on the piston; and the non-is mounted on a wrought iron plate which covers the frame, condensing engines, those which, having no condenser, allow the and is represented as at the instant when he is about to open steam, after it has acted on the piston, to escape into the at- the steam-valve 1, placed in the upper part of the fire-box z. mosphere. Such are the engines called Locomotive, which are In the lower part of the latter, is the fire, whence the flame and employed on railways. In these engines, however, the steam the products of combustion pass into the smoke-box Y, then is not lost, although allowed to escape into the atmosphere; for into the chimney-pipe, after having traversed 125 copper it is sent into the chimney to increase the draught, and thus tubes entirely immersed in the water of the generator. The to accelerate the combustion of the fuel; indeed, it may be generator, which is placed between the fire-box and the smokefairly doubted whether railways would have ever been so box, is made of brass, in the form of a cylinder of rather more generally adopted, as a medium of intercommunication and than a yard in diameter; it is surrounded by a wooden jacket, transit, had not this simple improvement of sending the steam which, by its weak power of conducting heat, resists the prointo the chimney of the locomotive, been discovered by some happy genius. cess of cooling by exposure to the external atmosphere. As cylinders placed one on each side of the fire-box; and in it proceeds from the generator, the steam is admitted into two these, by a system of valves similar to those described in the explanation of the double-acting engine, it acts alternately on the two sides of the pistons, the piston-rods transmitting the motion to the axle of the great wheels. The system of valves cannot be seen in the figure, because it is placed under the frame, between the two cylinders. After having acted on the pistons, the steam escapes into the chimney, and, as beforementioned, tends powerfully to increase the draught. The transmission of the motion of the pistons to the two great wheels is effected by two connecting rods, which, by means of cranks, connect the piston-rods with the axles of the wheels. As to the alternate motion of the valves in the valve-box of each cylinder, it is effected by means of eccentrics placed on the axle of the two great wheels. The water is renewed in the generator by the application of two pumps placed under the frame and worked by eccentrics. These pumps, by means of tubes, draw the water from a reservoir placed on the tender, or carriage situated immediately behind the locomotive, and following is a more detailed explanation of the different parts carrying the water and coal required for the journey. The of the engine.

Horse Power-In practical mechanics, the phrases labouring force, mechanical labour, permanent force, mechanical power, quantity of action, efficiency, etc., have been applied to the product of the effort made by a moving power measured in units of weight, by the space described in a unit of time, measured by a unit of length. It has been proposed to adopt, in this country, as a unit of the measure of labouring force (in French, travail), the force required to raise 1 pound weight, 1 foot high, in 1 second of time; and to call this unit, a dynamical unit, or simply a dynam. The dynamical unit, among the engineers in France, is the force required to raise 1 kilogramme, 1 metre high, in 1 second. Now, as the kilogramme is equal to 2.206 pounds weight, and the metre to 3.281 feet, the French dynam is 7.238 times the proposed dynam. In estimating the labouring force or mechanical power of a steam-engine, it has long been customary among engineers in this country to adopt as a dynamical unit, the labouring force or permanent power of a horse; and this evidently arose from the practice of employing horses in performing the labour now accomplished by steam, before the steam-engine was invented.

Watt found, from repeated experiments, that a horse treading a mill-path at the rate of 23 miles per hour, or 220 et per minute, will, on an average, raise 150 pounds weight by a cord hanging over a pulley, and suspended over a mine-1, and branches off into two pipes at the other extremity, in A, the copper pipe, which admits the steam at the extremity shaft; and this labour is equivalent to raising 33,000 pounds order to convey it to the two cylinders which contain the weight, one foot high per minute. Mr. Watt's steam-engines moving pistons. were constructed so as to work at the rate of 44,000 pounds raised one foot high per minute for each horse power; but in his estimate of their actual effect, he reduced this rate to 10,000 pounds raised one foot high per minute for each horse

engine. It communicates motion to the rod c, which is transB, the handle of the lever, for altering the course of the mitted to the valve-box.

c, the rod for altering the direction of the motion,

In the Great Exhibition at London, In 1851, a vast variety of steamengines, both for the purposes of manufacture and locomotion, were exhibited: these belonged principally to the high-pressure class, and motion was communicated to them by steam conveyed in pipes clothed with hairfelt under the flooring. These pipes derived their supply from five boilers, arranged, with boiler-house, near the north-west corner of the building. The system of clothing the pipes with thick hair-felt, and putting over that a casing of painted canvas, rendered it possible to carry high-pressure steam to a distance before thought to be impracticable. The pipes were supplied at intervals with globular water-traps, in which the water resulting from the condensation of the steam was collected, and from which it could readily be removed. The system of the non-radiation of heat was so complete, that no perceptible increase of temperature was experienced, even through the open flooring. The beam-engines of a former period were very generally replaced, in high-pressure engines, by those forms of arrangement in which a direct communication of power is made from the piston to the crank itself. To the latter class belong the steam-engines with vibrating or oscillating

LESSONS IN GEOLOGY.-No. LIII.

BY THOS. W. JENKYN, D.D., F.R.G.S., F.G.S., &c.

CHAPTER V.

THE CLASSIFICATION OF ROCKS.

SECTION VI.

THE WEALDEN STRATA.

In the south-east of Eugland there is a considerable district called the Weald of Kent, and the Weald of Sussex, and hence, the peculiar rocks which prevail in those regions have been called the WEALDEN.

The position of the Wealden strata is one of the most remarkable phenomena in the science of geology. They lie under the cretaceous beds, which were formed in a deep ocean, and they rest upon other rocks, the oolite, which were also formed in the sea; and yet all the beds of the Wealden have been formed by river water, and abound in fresh-water shells and fresh-water animals, with some indications of beds formed in brackish and sea-water.

I. THE LITHOLOGICAL CHARACTER OF THE

WEALDEN.

The Wealden rocks have been generally divided into three groups, which succeed in the descending order thus:

1. Weald clay, with thin beds of sand and shelly limestone. 2. Hastings sand, with clays and limy grits.

3. Purbeck beds, containing different kinds of limestones and

marls.

1. THE WEALD CLAY.

stone, alternating with slaty clay. All the sands are siliceous or flinty, and in colour are brownish or red, and sometimes yellowish. The sandstone is, in some places, a coarse conglomerate of pebbles. The fossil wood, the lignites, and the seams of coaly matter, that are seen in these sands, have misled some into the expectation of finding coal, as at Bexhill, in Surrey.

III. THE PURBECK BEDS.

Purbeck is a small peninsula, on the Dorsetshire coast, near Swanage, where there are fine sections of this group of the Wealden beds.

The Purbeck beds consist of slaty marls and of variegated limestone. What is called Purbeck marble consists chiefly of shells, some of them whole, but most of them much worn and comminuted. The shells are imbedded in a calcareous cement, which is sometimes very fine and crystalline, and sometimes has only the consistency of hard marl. As a marble it very much resembles the Petworth or Sussex marble.

Professor Edward Forbes divides the Purbeck beds into three groups, each marked by a peculiar species of organic remains, all of which are different from those found in the higher Wealdens of the Hastings sand and the Weald clay. The Purbecks are divided into the upper, the middle, and the lower beds.

The upper consists of purely fresh water beds of about fifty feet thick.

The middle consists of alternations of fresh water and brackish water and marine deposits, which, for distinction's sake, we will enumerate, seriatim, in the descending order :

1. Fresh-water limestone.

2. A limestone of brackish water full of Cyrena, and traversed by beds abounding in Corbulæ, Melaniæ, etc.-shells of brackish water.

The Weald clay is stiff in texture, dark brown on the sur-
face, but blue beneath, and contains concretional iron-stone.
It is finely exposed in the southern cliffs of the Isle of Wight,
in the form of slaty clay and limestone. When the slaty clay
is divided by a knife into lamina, its surface is found coated
with the shells of small cowries called Cypris Faba. The iron-thick, full of the shells of Ostrea distorta, with an Echinoderm,
5. A conspicuous bed called the "Cinder bed," twelve feet
stone of these beds was once extensively worked in the Wealds the first found in the Purbecks.
of Sussex and Kent, where the slags or cinders from the ancient
furnaces are still frequently met with.
Underneath this clay there are alternations of sands and
clays, which include a bed of limestone called Sussex marble
and Petworth marble, full of the shells of the Paludina vivi-

and other shells belonging to the sea.
3. A purely marine deposit, with Pecten, Modiola, Avicula,

water and partly in fresh water, in which many fish, and a
4. Beds of limestone and shale, formed partly in brackish
reptile called Macrorhyncus, are found.

para.

This clay bed surrounds nearly the whole of the Weald formation. It was the moistening and the wearing away of this bed of clay in the Isle of Wight, that occasioned the remarkable landslip which took place near Black Gang, in 1799. The thickness of this bed is, in Sussex, from 150 to 200 feet, but in the Weald of Kent it is 300 feet.

1. Ferruginous and fawn-coloured sands and sandstones, containing small portions of lignite and a stiff grey loam. 2. Sandstone.

3. A sandstone with concretionary courses of calciferous or limy grit.

A white sandstone, finely exposed in what is called "the

white sand rock" at Hastings.

6. Beds of clay and slate, and thin sandstones, containing lignites and silicified wood.

7. A sandstone without any concretions, but with numerous veins of argillaceous or clayey iron-stone.

8. Dark-coloured slate, with roundish masses of sandstone, and thin layers of lignites, and fragments of wood turned into

coal.

7. Thick siliceous beds of chert, filled with the above fossils.

forming the base of the middle Purbecks.
8. A very thin band of greenish shales, with marine shells,

This distinct enumeration is important, as it shows to you how the land, during the formation of the middle Purbecks, was sometimes subsiding and sometimes being elevated.

The lower has a series of beds like the middle group :1. A purely fresh-water marl, with species of Cypris, Valvata, etc., different from those of the middle group. It is about eighty feet thick.

2. Brackish water beds, seen at Meups Bay, with a species of Serpula, with Cypris, etc. The shales which contain the Cypris are much contorted and broken up. 3. The great DIRT BED, containing the roots and stumps of trees that grew upon the spot.

4. A fresh-water limestone, a bed about eight feet thick, bed No. 1 of this lower division. This forms the first sedicontaining shells of the same species as those enumerated in ments that were deposited by the Wealden river upon the

ancient oolitic rock.

The most remarkable bed in the whole of this Purbeck idea from the annexed wood-cut, series is the DIRT BED, No. 3, of which you will form some

which grew luxurious plants and trees; for in it, and growing The dirt bed is an accumulation of vegetable mould in from it, are found numerous trunks of cycadeous plants and The Hastings beds are, as a mass, principally arenaceous or growing in their native forests, with their roots in the veget coniferous trees, still erect, as if they had been petrified while gritty, but the lower part of the deposits, called by Dr. Mantell able soil, and their trunks rising into the limestone above. the Ashburnham beds, are composed of an argillaceous lime-Some of the trunks lie prostrate, but the greater part are up