If an order is considered as an assemblage of weight, and parts to support that weight, our experience immediately leads us to conceive a proper relation of those parts to their end. If the entablature be considered as the weight, then of course a certain form and size is demanded for the support of it, and in the base for the support of both. A plain stone, for instance, set on its end, has no proportion further than for the purpose of stability. If it appears firm it has all the proportions we desire or demand; and its form may be varied in a thousand ways, without interfering with our sense of its proportions. Place a column, or any other weight, on this stone, immediately another proportion is demanded, viz., its proportion to the support of this weight. The form supported has, however, no proportion further than is necessary for its stability. It may be more or less beautiful in point of form from other considerations, but not on account of its proportion. Above this again place an additional body, immediately the intermediate form demands a new proportion, viz., to the weight it supports; and the first, or base, demands also another proportion, in consideration of the additional weight which is thus imposed upon it. In this supposition, it is obvious, that the consideration of fitness alone leads us to expect a certain proportion among each of these parts. The parts are beautiful or pleasing, just as they answer to this demand; and where the parts are few, and experiments easy, it seems not difficult at last to arrive at that perfect proportion which satisfies the eye, as sufficient for the purpose of support or stability. If we leave, therefore, everything else out of consideration, the consideration of fitness alone seems to account both for the origin of such proportions in architecture, and for the plea

sure which attends the observation of them.

But granting that the doctrine of original beauty of proportion be abandoned as inconsistent with experience, and that of the influence of the expression of fitness be adopted, yet it may still be doubted if this is sufficient to account for the delight felt from the orders of architecture, or the uniform adherence to the established proportions.

It is acknowledged that the mere consideration of fitness is insufficient to account for the pleasure derived from the established orders. But it is observed, that this pleasure arises from their proportions; and that, in fact, when these proportions only are considered. the pleasure that is generally felt is not greater than that which we experience when we perceive in any great work the proper relation of means to an end.

emotion we feel, and readily attribute to the nature of architecture itself the whole pleasure which we enjoy. But besides these, there are other associations we have with these forms, that still more powerfully serve to command our admiration, for they are the Grecian orders; they derive their origin from those. times, and were the ornament of those countries which are most hallowed in our imaginations; and it is difficult for us to see them, even in our modern copies, without feeling them to operate upon our minds as relics of those polished nations where they first arose, and of that greater people by whom they were borrowed. While this species of architecture is attended with so many and so pleasing associations, it is difficult for even a man of reflection to distinguish between the different sources of this emotion; or in the moments when this delight is felt, to ascertain what is the exact protion of his pleasure which is to be attributed to these proportions alone; and two different causes combine to lead us to attribute to the style of architecture itself the beauty which arises from many other associations.

IMPRESSIONS OF MEDALLIONS, &C. IN COPPER. -Reduce finely pulverized oxide of copper in a current of hydrogen at a temperature below redness. Sift the powder through crape, and place it on the model, from four to five inches in thickness, and then press in a vice, or first with the hand and then with a hammer. The impressions obtained are perfect, but have little cohesion unless heated to redness previous to being exposed to the atmosphere. After this operation they acquire more tenacity than melted copper, and as their volume retracts, the impression becomes more distinct. Copper powder for this purpose is obtained far more easily, of better quality, and without loss of time, by precipitating a solution of sulphate of copper with zinc, and boiling. The precipitate of copper is then boiled for a few instants in weak sulphuric acid to remove the last traces of oxide or zinc; then washed with water, and dried in a tubulated retort over the water-bath by passing over it a current of hydrogen. This precipitate of copper has so strong an affinity for oxygen that it is difficult to prevent it from changing into protoxide; and if mixed with half an atomic weight of precipitated sulphur and rubbed together, they combine in a few instants with evolution of light forming proto-sulphuret of copper.

WHY WERE EARLY RAILWAYS CIRCUITOUS ?A history as interesting and extraordinary as a romance might be written of the difficulties The proportions of these orders, it is to be encountered and conquered by the early proremembered, are distinct subjects of beauty, jectors of railways. Nothing less than golden from the ornaments with which they are em- arguments of the purest mint would induce bellished, from the magnificence with which noble and gentle landholders to give assent to they are executed, from the purposes of ele-roads which trebled their estates in value: and gance they are intended to serve, or the vast loss of money and of time was incurred in scenes of grandeur they are destined to adorn. making those circuits which now excite our It is in such scenes, however, and with such wonder and regret, in order to allay the fears additions, that we are accustomed to observe of cities, lest conflagration-and universities, them; and while we feel the effect of these ac- lest contamination-should attend the near cidental associations we are seldom willing to approach of steam power.-Sidney's Railway examine what are the causes of the complex System.

Review.

A Catechism on the Steam-Engine. By JOHN
BOURNE, C. E.-London: J. Williams and
Co.

A CORRECT knowledge of the various minutiæ
connected with the construction and working
of steam-boilers and engines is of the utmost
importance at the present time, both to the
practical engineer who plans, and the mechanic
who constructs, those powerful instruments, in
which the maximum of power is concentrated,
as it were, into the minimum of space. The
name of Mr. Bourne is already well known to
the public as that of the author of a work on
the "Steam-Engine," which may justly be pro-
nounced as the most complete and practical
treatise of its description ever penned-we
refer to the quarto work published under the
implied editorship of the "Artisan Club."
From an inspection of the present work it is
easy to perceive that Mr. Bourne has striven
hard to advance his former reputation even
still more; and the result is that he has been
eminently successful, not only in producing a
book destined to become a requisite in the
hands of all connected with the subject on
which it treats, but also in securing an enviable
position in the foremost ranks of the engineer-
ing writers of the age.

tial explosion may be the result. The worst explosion is where the shell of the boiler bursts, but the collapse of a furnace or flue is also very disastrous generally to the persons in the engine room, and sometimes the shell bursts and the flues collapse at the same time; for if the flues get red-hot, and water be thrown upon them either by the feed pump or otherwise, the generation of steam may be too rapid for the safety valve to permit its escape with sufficient facility, and the shell of the boiler may in consequence be rent asunder. Sometimes the iron of the flues becomes highly heated in consequence of the improper configuration of the parts, which by retaining the steam in contact with the metal, prevents the access of the water: the bottoms of large flues upon which the flame beats down, are very liable to injury from this cause, and the iron of flues thus acted upon may be so softened that pressure of the steam. The flues of boilers the flues will collapse upwards with the may also become red-hot in some parts from the attachment of scale, which from its imperfect conducting power will cause the iron to be unduly heated; and if the scale be accidentally detached, a partial explosion may occur in conIt is found, however, that a sudden sequence. disengagement of steam does not immediately follow the contact of water with the hot metal, for water thrown upon red-hot iron is not immediately converted into steam, but assumes the spheroidal form and rolls about in globules over the surface. These globules, however high the temperature may be on which they are placed, never rise above the temperature of 205 degrees, and give off but very little steam; if the temperature of the metal be lowered, and comes into intimate contact with the metal, the water ceases to retain the spheroidal form, whereby a rapid disengagemeut of steam takes place. If water be poured into a very hot there will be scarce any steam produced so copper flask, the flask may be corked up, as long as the high temperature is maintained, but so soon as the temperature is suffered to fall below 350 degrees or 400 degrees, the spheroidal condition being no longer maintainable, steam is generated with rapidity, and the cork will be projected from the mouth of the flask with great force. One useful precaution against the explosion of boilers from too great an internal pressure, consists in the application of a steam guage to each boiler, which will make the existence of any undue pressure in any of the boilers immediately visible; and every boiler should have a safety valve of its own, the passage leading to which should have "The chief cause of boiler explosions is, no connection with the passage leading to any undoubtedly, too great a pressure of steam, or of the stop valves used to cut off the connecan insufficient strength of boiler; but many tion between the boilers; so that the action of explosions have also arisen from the flues the safety valve may be made independent of having been suffered to become red-hot. If the action of the stop valve. In some cases the safety valve of a boiler be accidentally stop valves have jammed, or have been carried jammed, or if the plates or stays be much worn by corrosion while a high pressure of steam is nevertheless maintained, the boiler necessarily bursts; and if from an insufficiency of water in the boiler, or from any other cause the flues become highly heated, they may be forced down by the pressure of the steam, and a par

The only fault we have to find with Mr. Bourne's present work is that it has been put into the form of a catechism; but even in this respect it may be doubted whether this is really one.

Our space will not allow us to extract largely, we will therefore select at random from the vast store of information with which the work abounds.

Steam-boiler explosions form, at the present time, a doubly interesting subject, and on which Mr. Bourne remarks as follows:

from their seats into the mouth of the pipe communicating between them, and the action of the safety valves should be rendered independent of all such accidents. Safety valves, themselves, sometimes stick fast from corrosion, from the spindles becoming bent, from a distortion of the boiler top with a high pressure,

in consequence of which the spindles become jammed in the guides, and from various other causes which it would be tedious to enumerate; but the inaction of the safety valves is at once indicated by the steam guage, and when discovered, the blow-through valves of the engine and blow-off cocks of the boiler should at once be opened, and the fires raked out. A cone in the ball of the waste steam-pipe to send back the water carried upwards by the steam should never be inserted; as in some cases this cone has become loose, and closed up the mouth of the waste steam pipe, whereby the safety valves being rendered inoperative the boiler was in danger of bursting. If the water be carried out of the boiler so rapidly by priming that the level of the water cannot be maintained, and the flues or furnaces are in danger of becoming red-hot, the best plan is to open every furnace door and throw in a few buckets full of water upon the fire, taking care to stand sufficiently to the one side to avoid being scalded by the rush of steam from the furnace. There is no time to begin drawing the fires in such an emergeney, and by this treatment the fires, though not altogether extinguished, will be rendered incapable of doing harm. If the flues be already red-hot, on no account must cold water be suffered to enter the boiler, but the heat should be maintained in the furnaces, and the blow-off cocks be opened, or the mudhole doors loosened, so as to let all the water escape; but at the same time the pressure must be kept quite low in the boiler, so that there will be no danger of the hot flues collapsing with the pressure of the steam. Plugs of fusible metal were at one time in much repute as a precaution against explosion, the metal being so compounded that it melted with the heat of high-pressure steam; but the device, though ingenious, has not been found of any utility in practice. The basis of fusible metal is mercury, and it is found that the compound is not homogeneous, and that the mercury is forced by the pressure of the steam out of the interstices of the metal combined with it, leaving a porous metal which is not easily fusible, and which is therefore unable to perform its intended function. In locomotives, however, and also in some other boilers, a lead rivet is inserted with advantage in the crown of the fire-box, which is melted out if the water becomes too low, and thus gives notice of the danger. All boilers in actual use should be proved at least once a year by forcing water into them by the hand feed-pump until the safety-valve is lifted, which should be loaded with at least twice the working pressure for the occasion. If a boiler will not stand this test, it is not safe, and either its strength should be increased or the working pressure should be diminished."

Mortar.

MORTAR is the calcareous cement used in building, compounded of burnt limestone and sand. Limestone or chalk, slowly calcined till all its carbonic acid gas is expelled, loses about 44 per cent. of its weight, and becomes what is called slacked lime, on the application of water, falling into a fine powder: suddenly formed into a stiff paste with water, this powder becomes hydrate of lime, assuming a solid form, and, as it attracts carbonic acid from the air, hardens, and constitutes the common builder's mortar. Hydrate of lime, without any other ingredient, whilst in a moist state, falls away and is dissolved in water. Alumina, silica, and manganese produce the same effects with lime, but in an inferior degree. If any substance in powder, which contains much iron, be added to this mortar, its hardness and compactness is much increased, and it also acquires the property of hardening under water, and is denominated Roman cement, or water mortar. In these combinations, if the lime should be in excess, it separates, either crystallising or forming stalactes; by which it appears that a definite proportion of the materials is necessary to form the best cement.

Various additions are made to mortar, for the purpose of increasing its hardness or tenacity, which for this purpose must be reduced to a fine powder, and the whole intimately kneaded together, and wrought to a smooth consistence: puzzolana, terras, iron ores, basalt, and other similar substances, in considerable variety, with sand of various descriptions. Common limestone (the chaux maigre, or poor limestone, of the French) consists of nearly pure carbonate of lime. It slacks freely, and produces white lime, and, with the addition of sand, good common mortar. The species of lime includes chalk, marble, &c. Chalk lime is seldom sufficiently burnt, and more quickly absorbs carbonic acid than stone lime. Silicious limestones give a buff colour. Those stones which are almost entirely constituted of pure carbonate, as is the case with respect to most of the varieties of marble or limestone called rich, when burned, slacked, and made into paste, will retain their softness under water, or otherwise excluded from the air, for almost any length of time; whilst those stones in which calcarious matter is found mixed with sand, silex, alumine, and iron, and which are called meagre limes, if treated in the same manner, quickly harden under water, and in time form a kind of freestone, which cannot be easily broken: from this circumstance it is called hydraulic lime. Mortar of this lime, when exposed to the air to dry, acquires a crumbly consistence; and these kinds of lean lime, which contain an excess of silex, are found unfit for hydraulic cement. The rich lime, exposed to the air, JENNINGS'S PATENT JOINTS.-An important mixed with a due proportion of sand, acquires improvement has been lately effected by Mr. a great degree of hardness. Puzzolanas are G. Jennings, of Great Charlotte-street, Black- either natural or artificial. The natural are friars-road, consisting in a new method of join- found in situations which have been acted upon ing pipes, &c., together without using solder. by subterraneous heat. They all consist of

We will, perhaps, shortly return to this book; but in the meantime advise all who can aflord it to purchase for themselves.