trived to open one way, to admit a fluid into a tube, but which shuts when pressed from the other, to prevent its return. In anatomy, a kind of membrane which opens in certain vessels to admit the blood, and shuts to prevent its regress. Valves, in blast machines, are essential in blast-conducting pipes; first, for shutting up the blast entirely; secondly, for diminishing and increasing it at pleasure. The first kind is needed where the blast is generated, for various purposes, by the same blast machine. The valves in use are, the sliding, the conical, and the trundle. The two first named are but little employed. If well made, the latter kind of valve is very useful. one end it has a handle, and, in many instances, a graded scale, which indicates the amount of air which passes through the valve, or, in other words, it shows the opening of the valve. At each tuyère or nozzle a valve is required, which serves either to shut off the blast entirely, or to regulate the passage of whatever amount is needed. At the nozzle-valve, a scale is very useful, partly for the purpose of adjusting the blast, and partly for that of fastening the handle of the valve, and keeping it in a certain position.

At

The laws which govern the construction of blast-pipes, valves, and tuyères, are summarily as follows: The interior of the blast conductors should be as smooth as possible, as an uneven surface causes great friction. The friction of the air is proportional to the length of the pipe, and to the density of the air which passes through it. It is proportional to the square of the speed of the air, and the reverse of the square of the diameter of the pipe. Obstructions caused by short bends in such pipes are inversely proportional to the angle of the bend, and are governed by the laws of hydrostatics. Sud

den contractions and expansions of the pipe occasion a whirling disturbance in the current of the air - a loss of power, or, what is the same, of blast. Valve, the safety, in pneumatics, a valve in a steam engine, to obviate the danger of explosion, by allowing the steam to escape when the pressure is raised beyond a certain weight

Valve-seat, the flat or conical surface

upon which a valve rests Vandyke Brown, a pigment hardly less celebrated than the great painter whose name it bears, is a species of peat or bog-earth, of a fine deep semi-transparent brown colour. The pigment so much esteemed and used by Vandyke is said to have been brought from Cassel; and this seems to be justified by a comparison of Cassel earth with the browns of his pictures. The Vandyke browns in use at present, appear to be terrene pigments of a similar kind, purified by grinding and washing over: they vary sometimes in hue, and in degrees of drying in oil, which they in general do tardily, owing to their bituminous nature, but are good browns of powerful body, and are durable both in water and oil. Vane, or Wind-vane, in navigation, a thin slip of bunting; a string of feathers, &c., stuck up to windward, to show the direction of the wind

Vanes. In Europe, the custom of placing vanes on church steeples is very old; and, as they were made in the figure of a cock, they have been thence denominated weathercocks. Vanning, removing the impurities from tin ore

Vaporization. The presence of moisture in the air is accounted for by a modification of the process of vaporization. Water evaporates, or is converted into steam (by steam we here mean the elastic vapour of water, which is always invisible;

what is commonly called steam, but properly cloud, is liquid water in a finely divided or powdered state, wafted like dust by currents of air or of steam, properly so called,) at all temperatures, until the whole space above it, whether containing air or not, is pervaded with watery vapour of a certain fixed density and elasticity, depending on the temperature, and connected therewith by certain laws. The elasticity or expansive tendency of a fluid is estimated by the number of pounds or ounces with which it presses on each square inch of surface that it touches; or by the number of inches of mercury that it will support, as on a barometer.

Steam can exist at any given temperature, and of such density as to have a certain fixed pressure, and no more; and (if there be water enough present) steam will be accumulated till it has this density; but no more can then be accumulated without raising the temperature and if the temperature be lowered, a portion of the steam will immediately become water, so that (occupying in this state some thousands of times less space than before) it may leave room for the remaining vapour to expand, till its expansive force is reduced to that which the new temperature can support. The pressure of steam is, therefore, always the same at the same temperature. At 212° its elastic force is equal to that of the atmosphere, and it will support a column of mercury 30 inches high, which is the reason that boiling requires this temperature in the open air, when the barometer is at 30 inches; but rather less or more, when the barometer stands lower or higher. Above this temperature it becomes high-pressure steam, which at 220° will support nearly 35 inches of mercury; at 230°, nearly 42 inches, and so on. But the steam which is thrown off

from the waters of the earth, from damp soil, from the foliage of plants, and even from ice and snow, has but a very small pressure. Steam at 32° will support only 0-200 of an inch of mercury: at 40°, 0.263 of an inch; at 50°, 0.375 of an inch; at 60°, 0.524, or rather more than half an inch of mercury; at 80°, it will support one inch, and so on. When the air contains as much vapour as can exist at the existing temperature, it is said to be saturated. If in this state it experience the smallest reduction of temperature, some of the vapour must immediately become liquid, assuming the form of cloud, fog, or rain. These effects depend on the cooling of the air below the temperature necessary to retain all its vapour. But when a solid body is cooled below this temperature, (the air remaining above it,) a different kind of deposition occurs, called dew, which does not fall in drops from the air, but grows, as it were, on the solid. Dr. Wells proved, by a most complete investigation of this subject, that instead of dew cooling bodies, as commonly supposed, it is their cooling which causes dew; and its formation even mitigates the cold, by the heat previously latent, which the steam gives out on condensing into water. The degree of heat at which dew begins to be formed is called the dew-point, and instruments called hygrometers have been invented to measure it. The difference between the temperature of the dew-point and the temperature of the atmosphere indicates the degree of dryness, which in this country seldom reaches 30°; that is, the temperature of the earth necessary to condense the vapour of the air is seldom 30° below the temperature of the air. In India it has been known to be 61° below it, and in Africa probably lower still.

If, while dew is forming, the earth continues to cool down until

it reaches the freezing-point, hoarfrost is formed. The beautiful figures seen in winter on the inner surface of window panes, cooled by the external air, are produced by these cold surfaces condensing the moisture of the warmer air within.

Varry, a term in heraldry, denoting the mixture of argent and azure together

Varry cuppy, a term applied to a fur of cups

Vat, a wooden tub, used to wash ores

and mineral substances in; a working-tub of any kind

Vault, in architecture, an arched roof, so contrived as that the several stones by their disposition shall support each other Veering, or Wearing, in navigation,

the operation to which a ship, in changing her course from one board to the other, turns her stern to windward, in opposition to tacking, wherein the bow is turned to the wind and the stern to leeward

Vein, a course of metal in a mine:

a rake vein is perpendicular, or nearly so; a pipe vein, nearly horizontal

Vein, in mining, to wash or cleanse a

small portion of ore in a shoal Velocimeter, an apparatus for measur

ing the rate of speed of machinery. When the velocity is uniform, the instrument is merely a measurer of distance; but this is not the case with a variable velocity, which requires a much more elaborate contrivance for its estimation. Such a velocity-measurer was constructed by Breguet, of Paris, under the direction of M.'Morin, the principle of which may be briefly explained as follows: A circular disc, covered with card or paper, is made to revolve with an uniform motion by means of clock-work, regulated by air-vanes upon this disc, a revolving pencil, whose motion is caused by and corresponds with that of the body whose variable

velocity is to be measured, describes a curved line; and from this curve, which results from a combination of the variable with the uniform motion, the velocity may be easily ascertained by processes and formulæ adapted to the purpose. One of these cards, with the curve traced on it by the piston of the Cornish steam engine at Old Ford, is engraved in the Transactions of the British Association for the Advancement of Science.' This beautiful and ingenious contrivance, by which spaces described in the tenthousandth part of a second may be easily discerned, is the invention of M. Poncelet, and was carried into execution by M. Morin.

The instrument, when put in order, was first tried at King's College, London, a variable motion being given by a small carriage made to descend an inclined plane. The correspondence of the velocity shown by the machine with that deduced by the known laws of dynamics, was such as to give great confidence in its accuracy. After a few minor alterations, suggested by frequent trials, it was removed to the East London Water-Works, and attached to the Cornish engine at work there, from which several diagrams were taken; and the velocities calculated from these have been expressed in the form of geometrical curves, whose abscissæ represent the spaces passed over by the piston of the engine, and whose ordinates indicate the corresponding velocities at the different points of the stroke.

Velocipede, a carriage which is capable of being propelled along a road by the muscular power of the rider acting upon treadles and levers which communicate with a cranked wheel axle

Velocity, in dynamics, is the ratio of the quantity of linear extension that has been passed over in a certain portion of time; or it is the ratio of the time that has been

employed in moving along a determinate extension.

When a man ascends vertically, his velocity is reduced to about one-half of his horizontal velocity, indicating that he acts against a double resistance; therefore, when a man ascending a ladder, carries a load, the maximum effect will take place when his ascending velocity is about onefourth of the velocity he can walk horizontally without a load.

A man of ordinary strength will not be able to walk, unloaded, at a quicker rate than 3 miles an hour, if this exertion is to be continued for 10 hours every day. Indeed, those who examine the subject with a view to a fair average, will find this to be about the extreme velocity that. can be continued, without injury, for any considerable time; therefore a man ought to move with half this velocity to produce a maximum effect; that is, at the rate of 1 mile an hour, which is about 2 feet per second.

But this supposes the whole load to be the useful effect, whereas part of it must consist of the apparatus employed to carry it, or the friction of the intermediate machine, or other circumstances of a like nature. About one-fifth of the velocity may be considered equivalent, at an average, to the force lost in friction, &c., in all cases; in many it will exceed onefifth.

Hence the maximum of useful effect will take place when the velocity is 2 feet per second, or about 11 furlongs an hour, continued for 10 hours each day.

Smeaton is said to have made numerous comparisons, from which he concluded that the mechanical power of a man is equivalent to 3750 tbs. moving at the velocity of one foot per minute; and taking this average to be near the true one, as there is reason to conclude it is, we have

Therefore, we make the average mechanical power of a man 31-25 lbs. moving at the velocity of 2 feet per second, when the useful effect the greatest possible; or half a cubic foot of water raised two feet per second; a very convenient expression for hydrodynamical inquiries.

If a man ascend a vertical ladder, according to a preceding remark, the velocity which corresponds to the maximum of useful effect will be 1 foot per second, and the load double that which he carries horizontally; consequently the average of useful effect is 62.5 tbs. raised one foot per second.

Bricklayers' labourers in London ascend ladders with a load of about 80 lbs. besides the hod; sometimes at the rate of one foot per second, but more frequently about 9 inches per second.

Ascending stairs is more trying to the muscles of the legs than ascending a ladder; and therefore the useful effect is less, till a person has become accustomed to this kind of labour: and it is also to be observed that the space moved over is increased, unnecessarily, except where the horizontal distance is part of the path over which the load is to be moved.

The force of a horse is, at an average, about equal to that of six men, according to various estimates; and the rate of travelling about the same, perhaps rather less than that of a man, when his exertion is continued for 8 hours: consequently the velocity corresponding to the maximum effect will be about 2 feet per second. Whence, the average mechanical power of a horse may be estimated at 187 lbs. moving with a velocity of 2 feet per second, or 3 cubic feet of water raised 24 feet per second; the day's work being 8 hours.

Velvet painting is the art of colouring on velvet with transparent liquid and other ready diluted colours, compounded and made up with various acids, alkalies, &c., according to their nature and qualities Venetian Red or Scarlet Ochre. True Venetian red is said to be a native ochre, but the colours sold under this name are prepared artificially from sulphate of iron, or its residuum in the manufacturing of acids. They are all of redder and deeper hues than light red, are very permanent, and have all the properties of good ochres. Prussian red, English red, and rouge de Mars, are other names for the same pigment.

Ventilation and warming of buildings is a twofold purpose that should

enter into the constructive design of all edifices intended for the residence or occasional congregation of human beings. The necessity for this purpose arises from the fact that the breathing of air (as one of the functions of animal life) renders it unfit for re-inspiration, the lungs retaining the vital properties and emitting the remainder, which consists of ingredients detrimental to health, and even destructive of life itself. In order to keep an apartment in a healthy and pleasant condition, fresh air should be constantly supplied at a temperature from 60° to 65°, and the vitiated air should be as constantly removed; and all the varied schemes which have been propounded for ventilating buildings have this common purpose of constant supply and removal. The vitiated air, on being emitted from the mouth, has a temperature between 80° and 90°; and as the universal effect of heat, manifested in the increase of temperature of the supply, 60° or 65° to 80° or 90°, is to expand and lighten, the vitiated air has a natural tendency to rise to the upper part of the room. To allow this action to proceed, it is evidently necessary that means for its escape at the top should be provided, and also that fresh air should be introduced at the lower portion of the apartment. The operation would, however, be nullified if the heated air, on emerging from the top of the room into a shaft or chimney intended to conduct it away, were met by a downward current of cold air; and it has therefore been deemed advisable to provide not only such a shaft or chimney, but also some means, by stoves or other apparatus, of artificially heating the air in the shaft, and thus assisting the escape of the foul air. And further, in order to secure the constant accession of fresh air and give sufficient impulse to it to overcome any tendency