forming the base of the pyramidal apex of the obelisk had been placed, a light hanging scaffold was formed, consisting of four strong wooden bearers, each 12 feet long, fitted to the slopes of the said course, and bolted together at four points, so as to form a square frame, having its ends projecting about 3 feet each way beyond the outside of the masonry. Planks were bolted down upon these projecting ends on three sides of the frame, but the fourth had a pulley attached to it for a rope and bucket, by which the workmen were to be drawn up and let down. This scaffold was held down and kept in its proper position by a pair of guy-ropes made fast below, on contrary sides of the obelisk, each branching out into two parts at top, like the capital letter Y, in order to act upon all the angles of the frame.

Mode of removing the Pole and Crossbeam.-As soon as the hanging scaffold was completed, all the materials necessary for finishing the remainder of the work were got up and piled upon it, after which, the upper part of the pole, having the cross-beam attached to it,was sawed off, and lowered down to the ground below, whilst the lower part of the pole was left standing, and is now buried within the masonry.

Mode of removing the Hanging Scaffold. The four bearers composing the frame of this scaffold were fixed together by four bolts, as before stated, each of which was fitted as an eye-bolt for receiving a rope at its lower end. Two of these bolts, at opposite angles, were screw-bolts, but the two others, at the alternate angles of the frame, were slipbolts with their upper ends keyed, and the head of each of these keys was provided with a ring for receiving a rope, and at their other ends, they themselves were secured by a second pair of smaller keys, as shown in fig. 4.

After the masonry was finished, the planks of the scaffold were disengaged

and lowered down, and at the same time ropes were made fast to the eyes of the four bolts, and to the rings of the two principal keys, the former being led straight down to the base of the obelisk, but the latter being first passed in a horizontal direction over pulleys at the ends of two of the bearers. The ropes from the bolts were passed through snatch-blocks, near the base of the obe

Fig. 4.

lisk, and from thence led out to such a distance that they might be acted upon by a windlass, without danger to the men. These preparations being taken, one active mason, who remained the last, disengaged the second pair of small keys from the first, and then descended by the bucket; after which the two principal keys were drawn out by means of the ropes fixed to their rings, as before described. The slip-bolts were then pulled out, by acting in like manner on the ropes attached to them.* This being done, the four bearers ceased to form a connected frame, being separated into two parts, each part consisting of two pieces of wood, held together at right angles to each other, in the form of an irregular cross, by their respective screwbolts.

The ropes attached to these bolts were then acted upon, and the parts of the scaffold pulled down, which descended along two opposite angles without injuring the masonry; for the projecting cornice, which was the most exposed part of it, had previously been covered with sods, and further protected by eight long poles, leaning against the

sides of the shaft above it.

* Two small lines, one of which is shown at g, in fig. 4, were used, in a very ingenious manner, to relieve these keys from the weight of their own ropes, which, if not thus counteracted, might have drawn them before the proper time, with great danger to the mason who remained last on the scaffold. As soon as this man had descended, these small lines, which had just sufficient strength to prevent an accident of this kind, were broken by a strong pull of the windlass, acting upon the key-ropes, by which the keys themselves were at the same time extracted.

Mr. Thomas Slacks, the inventor of the above ingenious arrangements, is an operative mason, who was previously employed as foreman of masons under the able engineer of the Carlisle and Newcastle railway, in constructing a remarkably fine skew-bridge over the small river Gelt, in Cumberland, in which the machine called a Hercules was used. Mr. Slacks states, that he took the idea of his own machinery solely from the Hercules, which he modified by changing a straight into a radiating movement. It will be seen, however, that the principle adopted by him is much more similar to that of the balance-crane used by Mr. Stevenson more than twenty years ago, in the erection of his justly celebrated work, The Bell Rock Light-house, Hence, in recommending Mr. Slacks to the favourable notice of the Society of Arts, the writer of this article has no wish to claim for him the merit of priority of invention, so far as regards the principle of his machinery. But it is hoped that the following observations will show, that Mr. Slacks' arrangements involve a considerable practical improvement upon the balance-crane. That very ingenious machine was constructed entirely of iron, of the very best workmanship, and the whole of the wheels, pinions, &c. acting upon it, formed an integral part of the crane itself, which was erected first upon the solid part of the masonry, and afterwards in each of the successive chambers of the light-house, previously to their being covered in. This machine could not have been used in the Malcolm Obelisk at all, because the hollow spaces allowed in the centre of that work were much too small to admit it; besides which, the expense of such a machine would have far exceeded the funds at the disposal of the managing committee. In fact, it is much too costly ever to be used at all, except in very important and arduous works, where nothing simpler will answer equally well, and where expense is a secondary object. Mr. Slacks' crane, on the contrary, was of the cheapest and simplest possible form, consisting chiefly of a larch-tree, cut in the neighbouring woods, and a short piece of square timber, besides some other very simple wood-work, iron-work, and ropes. In respect to his machinery, it consists of a crab and small windlass, such as every

builder must necessarily be provided with, and which, by not forming an integral part of his crane, but being entirely separated from it, saves superfluous expense; and, so far as regarded the peculiar nature of the Malcolm Obelisk, this arrangement also was much more convenient, since, by being worked from below, the machinery, as well as the men employed at it, were entirely out of the way of the masons whilst in the act of building. Moreover, by placing his crab below, Mr. Slacks gets rid of the necessity of using a suspended and movable weight as a counterpoise to the materials raised, which is so very essential a part of the balance-crane, that it has derived its name from it. In short, instead of that very ingenious but ex pensive machine, which certainly was much better adapted for the work of the Bell Rock Light-house than any other that could have been used, but which is only capable of being made by first-rate iron-founders and mechanists, Mr. Slacks has contrived an apparatus no less efficient for the purpose he had in view, but infinitely cheaper, and so much simpler, that the same might be fitted up by common country carpenters and black smiths in any part of the United Kingdom, or of the British colonies. In respect to his arrangements for finishing the work bythe hanging scaffold fixed upon the first course of the pyramidal apex of the obelisk, which scaffold was taken to pieces from below after all the workmen had descended, this part of his plan is still more ingenious, if possible, than the former, and appears to be perfectly original.

By the process that has been described, this obelisk was executed, without the smallest accident, in rather less than twelve months, in a manner highly creditable to the contractors, and satisfactory to the committee by whom they were employed.

KNIGHT'S ILLUMINATED ENGRAVING. Mr. Charles Knight, the indefatigable publisher to the Society for the Diffusion of Useful Knowledge, has just taken out a patent for a new invention in engrav ing, which, if but a small part of his glowing anticipations in regard to it should ever be fulfilled, will create quite

a revolution in the world of art. Certain it is, that the invention could not be in better hands, so far as the bringing it fairly before the public is concerned; Mr. Knight is not the man to sleep on his post, and, should the new process of "illuminated engraving" fail at last, it will assuredly be for no want of effort to give the world an opportunity of judging of its capabilities. Two or three series of coloured engravings, portraits and historical pieces, are already announced, as well as two series of maps, a species of illustration for which the new process is said to be peculiarly fitted. As the specification is not yet enrolled, it is not known in what way Mr. K. proposes to effect his object, but that it must be rapidly and easily attained is evident from the low prices at which these commencing series are to be published, two or three large coloured maps, for instance, at ninepence, or of the largest size for a shilling. It is conjectured that the whole of the colours are printed at once, probably from a block consisting of as many separate pieces as there may be colours required, the invention consisting in the method of unlocking those with facility to perform the inking, and locking them again to take the impression; this has, we believe, often been attempted, but never hitherto with success. Illuminated printing of engravings is of course already well-known, as witness the splendid specimens produced of late years by Mr. George Baxter, the eminent woodengraver, especially in his beautiful "Pictorial Annual." Mr. Knight cannot be expected to compete with such finished productions as these, the only objection to which consisted in the expensiveness of the method by which they were produced. Economy is to be the grand feature of the new plan, but it will hardly succeed unless due attention be paid at the same time to excellence in an artistical point of view.

CAOUTCHOUC SPRINGS.

Sir, I beg to inform J. R., whose suggestion for the employment of caoutchouc in the construction of "powerful springs" appeared in your last Number, (page 80,) that the peculiar properties of this curious substance render it wholly unfit for the purpose recommended. It

is perfectly true, that by confining a solid cylinder of this material in a suitable cavity, and using a very accurately fitted piston or plunger, "lateral expansion" might be prevented; there are other difficulties, however, not so easily got over, among others, to wit, the decomposition by heat, of the caoutchouc, which would be the certain consequence of powerful concussion. This is a very inconvenient effect, and I fancy it would be no easy task to find a practical method of preventing its occurrence.

It appears to me that nearly all the purposes to which the caoutchouc-rod spring would be applicable, are already fully met by the employment of spiral and other metallic springs, some very ingenious applications of which have recently been made in the buffers of railway-engines and carriages, and for some other purposes. For my own part, I cannot perceive what important advantages the application of caoutchouc in this form holds out, to compensate for the numerous inconveniences that would inevitably attend its employment. Under no circumstances would the re

sult of this arrangement be " a very effective spring," as J. R. would find should he take the trouble to experiment thereon.

I am, Sir, yours respectfully,
WM. BADDELEY.

June 26, 1838.

MILLS'S MERCURIAL PUMP-A
FALLACY.

Sir, I think it right to offer a few remarks on "Mills's Mercurial Pump," which you have "not considered unworthy of a notice in your excellent periodical;" vide No. 776, p. 189.

I know nothing of Mr. Grier, (lecturer on natural philosophy in the Baronial Hall,) to whom the working model of Mr. Mills's pump was presented; but I cannot compliment him on his discrimi nation, that he did not at once detect and point out to Mr. Mills, the thorough fallacy of his contrivance. On the contrary, we are told, "its action was so satisfactory to the lecturer and his class," that a publication of the invention was solicited.

In Mr. Mills's description, his pump is said to be "of the suction kind," but in the engraved illustration which accompanies it, is represented to be a lifting

force-pump. Mr. Mills is somewhat explicit in giving a limit for the length of the feed-pipe, viz., 30 feet; but he gives no idea of the length required for the play of the mercury in the cylinder, a circumstance that would interfere most prodigiously with the efficiency of the pump, if the feed-pipe approached any way near the assigned limit.

The close-topped intermediate moving cylinder, is directed to be so placed as to be capable of easy motion upwards and downwards, but without hinting at the manner in which this is to be effected, or by what means the parallelism is to be preserved, so as to secure the promised advantage of "little friction."

If we suppose a pump constructed upon this principle, in the very best manner possible, with a clear perpendicular lift of 26 feet, the working cylinder would require to be nearly three feet high; for on raising it, the mercury would fall on the outside and rise within, in proportion to the extent of vacuum induced. Again, on depressing the handle, the mercury would, in the first instance, return to a state of equilibrium, and then, if the pressure of a column of water, air-vessel, &c., has to be encountered, the mercury would fall in the inner, and rise in the outer cylinder, until the height of the column was equal to the opposing pressure. It is this play or oscillation of the mercury that I have before alluded to, and which would, in practice, render almost nugatory more of the motive force, than the friction of any ordinary well-made pump.

The second advantage claimed by Mr. Mills for his mercurial pump, is, "that for every inch stroke of the handle, the water will be raised one foot high!" Now, this effect depends entirely upon the relative sizes of the respective parts of the apparatus, without any reference to the fact of the pressure of one inch of mercury being about equal to one foot of water, a fact illustrative of the inconveniency to which I have adverted.

There are other objections to this form of pump to which I need not now advert, but only observe in conclusion, that this is by no means a solitary instance, even in recent times, of clever men being imposed upon by the fallacious character of models.

I remain, Sir, yours respectfully,
WILLIAM BADDELEY.

London, June 28, 1838.

FIRE-PROOF HOUSES. DE WITTE'S FIRE-PREVENTIVE COMPOSITION.

Sir, From the extreme interest I have ever taken in the best methods of preventing the occurrence of calamitous fires, and for arresting their progresswith the share I have had in the discussion of these topics in your pages-you may very well suppose I have not been inattentive to the recent proceedings of Mr. G. J. De Witte, with his Patent Firepreventive Composition. In a prospectus issued a short time since for the forma

tion of a 'Fire-preventive Company," the patentee very justly observes, that "the prevention of so dreadful a calamity as fire will always be considered of the first importance to society, and the discovery of a composition which effectually renders timber and other materials indestructible by that element, will form a new era in the building of houses, shipping, and every erection in which wood is employed.' He then goes on to state, that "the experiments which have been already submitted to the public, have convinced the scientific world of the extensive utility of this invention, and of the impossibility of burning timber when covered with this preparation. By a cheap and easy process, walls, ceilings, partitions, staircases, &c., can be covered with this fire-proof material. It is alike applicable to houses under repair as well as when building,-a slight coating being sufficient to secure the woodwork from burning."

Having been informed that the patentees were preparing a house in Dorsetstreet, Clapham-road, for an experimental illustration of the perfect efficacy of their plan, I took an opportunity of inspecting their progress. The house, which was a small one, in connection with another adjoining, had been built in the usual way, with the intention of being fitted up in the ordinary style; it had a very liberal allowance of timber in its construction, but was a mere shell when taken up by Mr. De Witte and his colleagues, who proceeded to protect the boards and other timbers, the floors, ceilings, stairs, and wood-work generally, with their patent fire-proof composition, without in any way disturbing the original foundation or superstructure.

Their composition in appearance very closely resembles grey or slate-colored mortar; it is exceedingly pleasant to

work, dries extremely hard, is free from expansion or contraction under all variations of temperature, and preserves its cohesive properties to the last. When dry it is susceptible of as fine a polish as marble, and forms a better ground for the reception of paint than any other substance I am acquainted with; it can be stuccoed with ornamental devices in the usual manner, in accordance with the taste of the architect or builder.

The preventive process having been completed, Wednesday, the 6th of June, was appointed for the experiment, which was to put to a conclusive test the efficacy of the preparation. A great crowd collected near the spot, which was favourable for the accommodation of a large assemblage, on account of the open space afforded by the fields in the vicinity. An inclosure was made behind the premises for those persons who had been invited by the Directors. The company assembled, consisted of men of science, practical builders, several directors of insurance companies, and persons of importance and property, all of whom appeared to take a deep interest in the result of the experiment.

The upper floor was covered with shavings in great abundance, to which a number of deal planks were subsequently added. The first floor front room was fitted up as a chamber, with bed and furniture, chairs, tables, &c., as nearly as possible in the usual style of furniture, although, perhaps, less valuable, certainly not less combustible; on the contrary, this floor was likewise embedded with, at least, eighteen inches deep of dry shavings, with a plentiful supply of planks, &c. In consequence of the suggestion of some of the visitors, (who, on going over the premises had objected to the composition being exposed on the surface of the floor as offensive to the eye,) the floor, which had been prepared with the non-conducting composition, was covered over with a veneering of thin deal, which caused the flooring to present the usual appearance. Soon after two o'clock the scientific incendiary perpetrated an act which would have brought certain destruction upon any ordinary building; the shavings, wood, &c., on the upper floor were first fired, and burned with great rapidity, but laying low upon the floor, the appearances to those persons on the outside of the build

ing was not such as to prove altogether satisfactory. On igniting the combustible, in the first floor furnished room, however, a different effect was produced; the shavings, as a matter of course, were soon in a blaze, the bed and other furniture rapidly took the infection, and the windows being without sashes, a strong wind blew through the house, which caused the flames to rage with a degree of fury one might, have supposed to be altogether beyond the ordinary means of relief. The immense volume of fire that poured from the window, produced a semblance of what might, in common. parlance, be designated "a dreadful fire." At this moment the interest of the spectators was raised to the highest pitch: the furniture, &c., was entirely consumed, but the flames did not communicate to the adjoining apartment npon the same floor, nor to those above or below it. Fully proving what it was the object of these experiments to establish, viz., that fire can be confined to the apartment in which it originates by means of this composition, which effectually prevents contagion.

The only sign of injury to any portion of the building was exhibited by a lintel over one of the windows, the wood-work. of which being charred by the intense heat directed upon it, a small quantity of gas escaped through a minute fissure in the composition and burned for a few seconds only. The other apartments of the house were then subjected, in succession, to tests equally severe, by heaping shavings on the floor, which were fired, and more fuel added, until each of them might have been compared to the atmosphere of so many ovens; the result, in every case, proving equally successful.

Some iron hooping had been introduced into the ceiling of the first floor room, the expansion of which, by the excessive heat it was subjected to, caused the displacement of a considerable portion of the plastering; but the compo sition remained entire, and duly performed its office as was proved by several parcels of gunpowder which had been deposited between the ceiling and the floor above remaining untouched. This effect of iron hooping, is exactly that which has been anticipated by Mr. Cade, in his letter at page 184 of your last No., and, I think, Mr. C. will be