Chapter 6

The mould, when finished, is dried in a stove. Under such circumstances it often happens that there forms upon the surface of the plaster a hard crust which, although it is of no importance as regards the outside of the mould, is prejudicial to the interior because it considerably diminishes its absorbing power. This trouble may be avoided by coating the surfaces that it is necessary to preserve with clear liquid paste; but Mr. Renard advises that the mould be closed hermetically, so that the interior shall be kept from contact with warm air. In this way it is possible to prevent the plaster from hardening, as a result of too quick a desiccation. I now come to the operation of moulding. In the very first place, it is necessary to examine whether it is well to adopt the arrangement by pressure of air or by vacuum. The form of the objects will determine the choice. A very open piece, like a bowl, must be moulded by vacuum, on account of the difficulty of holding the closing disk in place if it be of very large dimensions. The same is the case with large vases of wood form. On the contrary, an elongated piece tapering from above is more easily moulded by pressure of the air, as are also ovoid vessels 16 to 20 inches in height. In any case it must not be forgotten that the operation by vacuum should be preferred every time the form of the objects is adapted to it, because this process permits of following and directing the drying, while with pressure it is impossible to see anything when once the apparatus is closed.

_Moulding by Pressure of the Air._--The plaster mould having been put in place upon the mould board, and the liquid paste having been long and thoroughly stirred in order to make it homogeneous, and get rid of the air bubbles, we open the cock that puts the paste reservoir in communication with the lower part of the mould, care having been taken beforehand to pour a few pints of water into the bottom of the mould. The paste in ascending pushes this water ahead of it, and this slightly wets the plaster and makes the paste rise regularly. When the mould is entirely filled, the paste is still allowed to flow until it slightly exceeds the upper level, and, spreading out over the entire thickness of the plaster, forms a sort of thick flange. The absorption of the liquid begins almost immediately, and, consequently, the level lowers. A new quantity of paste is introduced, and we continue thus, in regulating its flow so as to keep the mould always full. This operation is prolonged until the layer is judged to be sufficiently thick, this depending upon the dimensions, form, or construction of the vessel. The operation may take from one to five hours.

The desired thickness having been obtained, it becomes a question of allowing the paste to descend and at the same time to support the piece by air pressure. The flange spoken of above is quickly cut, and the paste is made to rise again for the last time, in order to form a new flange, but one that this time will be extremely thin; then a perforated disk designed for forming the top joint, and acting as a conduit for the air, is placed upon the mould. This disk is fastened down with a screw press, and when the apparatus is thus arranged the eduction cock is opened, and the air pump maneuvered.

If the flange did not exist, the air would enter between the mould and the piece at the first strokes of the piston, and the piece would be inevitably broken. Its object, then, is to form a hermetical joint, although it must at the same time present but a slight resistance, since, as soon as the liquid paste has flowed out, the piece begins to shrink, and it is necessary that at the first movement downward it shall be able to disengage itself, since it would otherwise crack.

As soon as the piece begins to detach itself from the mould the air enters the apparatus, and the pressure gauge connected with the air pump begins to lower. It is then necessary, without a moment's loss of time, to remove the screw press, the disk, and the upper part of the mould itself, in order to facilitate as much as possible the contraction of the piece. Finally, an hour or an hour and a half later, it is necessary to remove the lower part of the mould, this being done in supporting the entire affair by the middle. The piece and what remains of the mould are, in reality, suspended in the air. All these preparations are designed to prevent cracking.

_Moulding by Vacuum._--The operation by vacuum follows the same phases as those just described. It is well, in order to have a very even surface, not to form a vacuum until about three hours after the paste has been made to ascend. Without such a precaution the imperfections in the mould will be shown on the surface of the object by undulations that are irremediable.

The first flange or vein must be preserved, and it is cut off at the moment the piece is detached.

Moulding by vacuum, aside from the advantages noted above, permits of giving the pieces a greater thickness than is obtained in the pressure process. According to Mr. Renard, when it is desired to exceed one inch at the base of the piece (the maximum thickness usually obtained), the operation is as follows: The piece is moulded normally, and it is supported by a vacuum; but, at the moment at which, under ordinary circumstances, it would be detached, the paste is made to ascend a second time, when the first layer (already thick and dry) acts as a sort of supplementary mould, and permits of increasing the thickness by about 2/5 of an inch. The piece is held, as at first, by vacuum, and the paste is introduced again until the desired thickness is obtained.

Whatever be the care taken, accidents are frequent in both processes. They are due, in general, to the irregular contraction of the pieces, caused by a want of homogeneousness in the plaster of the moulds. In fact, as the absorption of the water does not proceed regularly over the entire surface of the piece, zones of dry paste are found in contact with others that are still soft, and hence the formation of folds, and finally the cracking and breaking of the piece. The joints of the moulds are also a cause of frequent loss, on account of the marks that they leave, and that injure the beauty of the form as well as the purity of the profile.

Mr. Renard has devised a remedy for all such inconveniences. He takes unglazed muslin, cuts it into strips, and, before beginning operations, fixes it with a little liquid paste to the interior of the mould. This light fabric in no wise prevents the absorption of the water, and so the operation goes on as usual; but, at the moment of contraction, the piece of porcelain being, so to speak, supported by the muslin, comes put of the mould more easily and with extreme regularity. Under such circumstances all trace of the joint disappears, the imperfections in the mould are unattended with danger, and the largest pieces are moulded with entire safety. In a word, we have here a very important improvement in the process of moulding. The use of muslin is to be recommended, not only in the manufacture of vases, but also in the difficult preparation of large porcelain plates. It is likewise advantageous in the moulding of certain pieces of sculpture that are not very delicate, and, finally, it is very useful when we have to do with a damaged mould, which, instead of being repaired with plaster, can be fixed with well ground wet sand covered with a strip of muslin.

_Drying of the Moulded Pieces._--When the moulded pieces become of a proper consistency in the mould, they are exposed to the air and then taken to the drying room. But, as with plaster, the surface of the paste dries very quickly, and this inconvenience (which amounts to nothing in pieces that are to be polished) is very great in pieces that carry ornaments in relief, since the finishing of these is much more difficult, the hardened paste works badly, and frequently flakes off. In order to remedy this inconvenience, it suffices to dust the places to be preserved with powdered dry paste.--_Revue Industrielle._

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PHOTO-TRICYCLE APPARATUS.

This consists of a portable folding camera, with screw focusing arrangement, swing back, and an adapter frame placed in the position of the focus screen, allowing the dark slide to be inserted so as to give the horizontal or vertical position to the dry plate when in the camera. To the front and base-board a brass swiveled side bar, made collapsible by means of a center slot, is attached by hinges, and this renders the camera rigid when open or secure when closed. The base-board is supported on a brass plate within which is inserted a ball-and-socket (or universal joint in a new form), permitting the camera to be tilted to any necessary angle, and fixed in such position at will. The whole apparatus is mounted upon a brass telescopic draw-stand, which, by means of clamps, is attached to the steering handle or other convenient part of the tricycle, preferably the form made by Messrs. Rudge & Co., of Coventry, represented in the cut.--_Photo. News._

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A PHOTO PRINTING LIGHT.

A printing frame is placed in the carrier, and exposed to the light of a gas burner kept at a fixed distance, behind which is a spherical reflector. The same frame may be used for other purposes.-_Photographic News._

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A NEW ACTINOMETER.

A selenium actinometer has been described in the _Comptes Rendus_ in a communication from M. Morize, of Rio de Janeiro. The instrument is used to measure the actinic power of sunlight when the sun is at various altitudes; but the same principle is applicable to other light sources. The sensitive part of the apparatus consists of a cylinder formed of 38 disks of copper, isolated from each other by as many disks of mica. The latter being of smaller diameter than the copper disks, the annular spaces between the two are filled with selenium, by the simple process of rubbing a stick of this substance over the edges, and afterward gently warming. The selenium then presents a grayish appearance, and is ready for use. Connection is made by conductors, on opposite sides, with the odd and even numbers of the disks, which diminishes the resistance of the selenium. The cylinder thus formed is insulated by glass supports in the inside of a vacuum tube, for the purpose of preserving it from the disturbing influence of dark rays. The whole is placed upon a stand, and shielded from reflected light, but fully exposed to that which is to be measured for actinic intensity. If now a constant current of electricity is passed through the apparatus, as indicated by a galvanometer, the variations of the latter will show the effect produced upon the selenium. A scale must be prepared, with the zero point at the greatest possible resistance of the selenium, which corresponds with absolute darkness. The greatest effect of the light would be to annul the resistance of the selenium. Consequently, the cylinder must be withdrawn from the circuit to represent this effect; and the maximum deviation of the galvanometer is then to be observed, and marked 100. By dividing the range of the galvanometer thus obtained into 100 equal parts, the requisite actinometric scale will be established. In practice, the Clamond battery is used to supply the constant current required.

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ASTRONOMICAL PHOTOGRAPHY.

During the last few years, or rather decades of years, it has become rather a trite saying that to advance far in any branch of physical research a fair proficiency in no inconsiderable number of the sister sciences is an absolute necessity. But if this is true in general, none, I think, will question the assertion that a proficient in any of the physical sciences must be fairly conversant with photography as a science, or at least as an art. If we take for example a science which has of late years made rapid strides both in Europe and America, the science of astronomy, we shall not have far to go to find convincing proof that a great portion of the best work that is being done by its votaries is effected by the aid of photography. One eminent astronomer has quite lately gone so far as to declare that we no longer require observers of the heavens, but that their place can be better supplied by the gelatine plate of the photographer; and his words have been echoed by others not less able than himself. "Abolish the observer, and substitute the sensitive plate," is a sensational form of expressing the revolution in observational astronomy that is taking place under our eyes; but, although it suggests a vast amount of truth, it might leave upon the mind an exaggerated impression inimical to the best interests of science.

The award of the highest distinction in astronomy, the gold medal of the Royal Astronomical Society, two years in succession, to those who have been most successful in celestial photography is no doubtful sign of the great value attached to such work. Last year it was Mr. Common who received the highest testimony of the merit due to his splendid photographs of the nebula of Orion; and this year Dr. Huggins, who has drawn much attention to celestial photography, by his successful attempts to picture the solar corona in full daylight, has received a similar acknowledgment of his labors in photographing the spectra of stars and comets and nebulæ.

An adequate idea of the progress astronomy is now making by aid of photography can only be formed by a comprehensive view of all that is being at present attempted; but a rapid glance at some of the work may prepare the way for a more thorough investigation. A few years since, the astronomers who had advanced their science by aid of photography were few in number, and their results are soon enumerated. Some good pictures of the solar corona taken during solar eclipses, a series or two of sun-spot photographs, and a very limited number of successful attempts made upon the moon, and planets, and star clusters, were all the fruits of their labors. But now each month we learn of some new and efficient laborer in this field, which gives promise of so rich a harvest.

Each day the sun is photographed at Greenwich, at South Kensington, in India, and at the Physical Observatory of Potsdam, and thus a sure record is obtained of all the spots upon its surface, which may serve for the study of the periodicity of its changes, and for their probable connection with the important phenomena of terrestrial magnetism and meteorology. In France the splendid sun-pictures obtained by Dr. Janssen at the Physical Observatory of Meudon have thrown into the shade all other attempts at a photographic study of the most delicate features of the solar surface.

Dr. Huggins has shown that it is possible to obtain a daily photographic record of the solar prominences, and only lately he has secured results that justified a special expedition to the Alps to photograph the sun's corona, and he has now moved the Admiralty to grant a subsidy to Dr. Gill, the government astronomer at the Cape, by aid of which Mr. Woods can carry on the experiments that were so encouraging last summer in Switzerland.

We may, then, reasonably hope to obtain before long a daily picture of the sun and a photographic record of its prominences, and even of a certain portion of the solar corona; but the precious moments of each solar eclipse will always be invaluable for picturing those wondrous details in the corona that are now shown us by photography, and which can be obtained by photography alone.

Again, how very much is to be learnt in solar physics from the marvelous photographs of the sun's spectrum exhibited last summer by Professor Rowland; photographs that show as many as one hundred and fifty lines between H and K, and which he is still laboring to improve! The extension, too, of the visible solar spectrum into the ultra-violet by Corun, Mascart, and others, adds much to our knowledge of the sun; while the photographs of Abney in the ultrared increase our information in a direction less expected and certainly less easy of attainment. Both these extensions we find most ably utilized in the recent discussion of the very interesting photographs of the spectra of the prominences and of the corona taken during the total eclipse of May 18, 1882; and the photographic results of this eclipse afford ample proof that we can not only obtain pictures of the corona by photography that it would be impossible otherwise to procure, but also that in a few seconds information concerning the nature of the solar atmosphere may be furnished by photography that it would otherwise take centuries to accumulate, even under the most favorable circumstances.

The advantages to be gained by accurate photographs of the moon and planets, that will permit great enlargements, are too obvious to call for lengthened notice in such a rapid sketch as the present; for it is principally in the observation of details that the eye cannot grasp with the required delicacy, or with sufficient rapidity, that photography is so essential for rapid and sure progress.

Like the sketches of a solar eclipse, the drawings that are made of comets, and still more of nebulæ, even by the most accomplished artists, are all, to say the least, open to doubt in their delicate details. And the truth of this is so obvious, that it is the expressed opinion of an able astronomer that a single photograph of the nebula of Orion, taken by Mr. Common, would be of more value to posterity than the collective drawings of this interesting object so carefully made by Rosse, Bond, Secchi, and so many others.

Another most important branch of astronomy, that is receiving very great attention at present, is the mapping of the starry heavens; and herein photography will perhaps do its best work for the astronomer. The trial star map by the brothers Henry, of a portion of the Milky Way, which they felt unable to observe satisfactorily by the ordinary methods, is so near absolute perfection that it alone proves the immense superiority of the photographic method in the formation of star maps. Fortunately this subject, which is as vast as it is fundamental, is being taken up vigorously. The Henries are producing a special lens for the work; Mr. Grubb is constructing a special Cassgrain reflector for Mr. Roberts of Maghull; and the Admiralty have instructed Mr. Woods to make this part of his work at the Cape Observatory, under the able direction of Dr. Gill. Besides star maps, clusters, too, and special portions of the heavens are being photographed by the Rev. T.E. Espin, of West Kirby; and such pictures will be of the greatest value, not only in fixing the position at a given date, but also aiding in the determination of magnitude, color, variability, proper motion, and even of the orbits of double and multiple stars, and the possible discovery of new planets and telescopic comets.

Such are some of the many branches of astronomy that are receiving the most valuable aid at present from photography; but the very value of the gift that is bestowed should make exaggeration an impossibility. Photography can well afford to be generous, but it must first be just, in its estimate of the work that has still to be done in astronomy independently of its aid; and although the older science points with just pride to what is being done for her by her younger sister, still she must not forget that now, as in the future, she must depend largely for her progress, not only on the skill of the photographer and the mathematician, but also on the trained eye and ear and hand of her own indefatigable observers.--_S.J. Perry, S.J., F.R.S., in Br. Jour. of Photography._

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ELECTRICITY AS A PREVENTIVE OF SCALE IN BOILERS.

The mineral sediment that generally sticks to the sides of steam boilers, and the presence of which is fraught with the utmost danger, resulting in many instances in great injury to life and property, besides eating away the substance of the iron plate, was referred to in a paper lately read by M. Jeannolle before the Paris Academy of Sciences, in which the author described a new method for keeping boilers clean. This method is as follows:

The inside of a steam boiler is placed, by means of piles of a certain power, in reciprocal communication, the current passing at one end through positive, and at the other through negative, wires. In incrusted steam boilers, at a temperature ranging from 212° to 300° Fahr., and a pressure of from 30 to 90 lb. to the square inch, the current thus engendered decomposes the accumulated salts, and precipitates them, from which they may easily be removed, either by means of a special siphon or by means of some other mechanical process. When boilers are free from fur, and where it is intended to keep them free from such, a continuous current may be set up, by means of which the sedimentary salts may be decomposed, and a precipitate produced in a pulverized form, which can be removed with equal facility.

From a series of minute experiments made by M. Jeannolle, it appears that in order to render the various actions of electricity, perfect, it is necessary to coat either with red lead or with pulverized iron, or with any other conductor of electricity, an operation which must be repeated whenever the boiler is emptied with a view to cleaning out. The above system Is being advantageously applied in Calais for removing the incrustations of boilers. The two poles of a battery of ten to twelve Bunsen elements are applied to the ends of the boilers, and after thirty to forty hours the deposits fall from the sides to the bottom. When a boiler has been thus cleared, the formation of new deposits may be prevented by applying a much less energetic current under the same conditions.

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ALPHABET DESIGNED BY GODFREY SYKES.

Among the many designs which have been issued by the South Kensington Museum authorities is the alphabet which we have illustrated here to-day. The letters appear frequently among the decorations of the museum buildings, especially in the refreshment rooms and the Ceramic gallery, where long inscriptions in glazed terra cotta form ornamental friezes. The alphabet has also been engraved to several sizes, and is used for the initial letters in the various official books and art publications relating to the museum, which are published by the Science and Art Department.--_Building News._

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OLD WROUGHT IRON GATE.

This gate forms the entrance to Scraptoft Hall, a building of the eighteenth century, now the seat of Captain Barclay, and which stands at about five miles from Leicester, England.--_The Architect._

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BRIEF SANITARY MATTERS IN CONNECTION WITH ISOLATED COUNTRY HOUSES.[4]

[Footnote 4: Read before the Boston Society of Civil Engineers, April 1884 _Journal A. of E. Societies_.]

By E.W. BOWDITCH, C.E.

I am unable to tell you what is generally considered the best practice, for I am not sure there are any definitely established rules; therefore I can only explain _my_ ways of doing such work, which, though I try to make as complete and at the same time as simple as possible, I know to be far from perfect.

Plumbing and drainage work has grown up unconsciously with my landscape gardening, and not finding any texts or practice that seemed wholly satisfactory, I have been forced to devise new arrangements from time to time, according to the requirements of the case in hand.