Chapter 5

At Fort Wagner, a sand work built during our war, Gen. Gillmore estimated that he threw one pound of metal for every 3.27 pounds of sand removed. He fired over 122,230 pounds of metal, and one night's work would have repaired the damage. The new fifteen inch pneumatic shell will contain 600 pounds of blasting gelatine, and judging from the German experiments at Kummsdorf, which I have cited, one of these fifteen inch shells would throw out a prodigious quantity of sand; either 500 pounds to one of shell, or 2,000 pounds to one of shell, according as the estimate of Gen. Abbot or of Capt. Zalinski is used. The former considers that the radius of destructive effect increases as the square root of the charge; the latter that the area of destructive effect for this kind of work is directly proportional to the charge.

The effect of the high explosives upon horizontal armor is very great; but we have yet to learn how to make it shatter vertical armor. No fact about high explosives is more curious than this, and there is no theory to account for it satisfactorily. As previously stated, the French have found that four inches of vertical armor is ample to keep out the largest melenite shells, and experiments at Annapolis, in 1884, showed that masses of dynamite No. 1, weighing from seventy-five to 100 pounds, could be detonated with impunity when hung against a vertical target composed of a dozen one inch iron plates bolted together.

In conclusion, I may say that in this country we are prone to think that the perfection of the methods of throwing high explosives in shell is vastly in favor of an unprotected nation like ourselves, because we could easily make it very uncomfortable for any vessels that might attempt to bombard our sea coast cities.

This is true as far as it goes, but unfortunately the use of high explosives will not stop there. I lately had explained to me the details of a system which is certainly not impossible for damaging New York from the sea by means of dynamite balloons. The inventor simply proposed to take advantage of the sea breeze which blows toward New York every summer's afternoon and evening. Without ever coming in sight of land, he could locate his vessel in such a position that his balloons would float directly over the city and let fall a ton or two of dynamite by means of a clock work attachment. The inventor had all the minor details very plausibly worked out, such as locating by means of pilot balloons the air currents at the proper height for the large balloons, automatic arrangements for keeping the balloon at the proper height after it was let go from the vessel, and so on. His scheme is nothing but the idea of the drifting or current torpedo, which was so popular during our war, transferred to the upper air. An automatic flying machine would be one step farther than this inventor's idea, and would be an exact parallel in the air to the much dreaded locomotive water torpedo of to-day. There seems to be no limit to the possibilities of high explosives when intelligently applied to the warfare of the future, and the advantage will always be on the side of the nation that is best prepared to use them.

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THE MANUFACTURE AND USE OF PLASTER OF PARIS.

It has long been a familiar fact that gypsum yields on baking a material which possesses the power of setting with water to a firm mass, this setting being accomplished much more quickly than is the case with mortar.

The explanation of the setting of plaster was first given by Lavoisier, who pointed out that gypsum is an hydrated salt, and that the set plaster is in fact gypsum reformed, the change brought about by baking being merely loss of water of crystallization. The beds of gypsum of most importance both formerly and at the present time in the plaster manufacture occur in the neighborhood of Paris in the lower tertiary formation. Different beds differ (1) in respect of character and quantity of admixed materials and (2) in the structure of the gypsum itself. With regard to the first point, some deposits contain a notable proportion of carbonate of lime, a fact which under certain circumstances may considerably influence the character of the plaster. In the matter of structure two principal varieties occur (1) granular and (2) fibrous. Further, hardness of the granular kind varies considerably. These differences of structure in the original material appear to exercise an influence on the properties of the plaster. Thus according to Payen the plaster formed from the granular variety sets more gradually than that derived from the fibrous, and forms a denser mass. The softer kinds of the granular gypsum are those principally used in the production of plaster for the moulds of potteries.

In the old fashioned process which is still employed for making the common kinds of plaster, the material is exposed to the direct action of flame. Large lumps are placed in the lower part of the furnace, above them smaller lumps, and, after the heating has been carried on for some time, finely divided material is filled in at the top. The outer portion of the larger lumps is always overburnt, and in the upper part of the furnace the presence of shining crystalline particles generally indicates the fact that some gypsum has remained unchanged. Provided that the amount of unburnt and overburnt material does not exceed about 30 per cent. of the total, the plaster is suitable for many applications.

It was early observed that set plaster could be revivified by a second baking, but attempts in this direction were not uniformly successful, it being found that the dehydrated substance in some cases refused to set with water. It behaved in fact similarly to the natural anhydrous calcium sulphate which is unaffected by water. These failures were found to be due to the employment of too high a temperature, and such plaster was termed _dead burnt_. Although this fact was ascertained long ago, yet ignorance of what had already been done has probably been the cause of many disappointments in attempts at revivification which have been made from time to time by persons unacquainted with the history of the subject.

The view generally adopted with regard to the theory of these processes is that plaster consists of anhydrous calcium sulphate, CaSO4, in a condition probably amorphous, different from that of natural crystallized CaSO4, known to mineralogists under the name of anhydrite. By the influence of a high temperature it appears probable that a molecular change is gradually induced with production of a crystalline structure, and probably an increase of specific gravity, resulting in the artificial reproduction of the mineral anhydrite. No determination appears to have been published of the specific gravity of plaster prepared by complete baking at a low temperature. The theory is, however, confirmed by the results obtained by workers on the subject of mineralogical synthesis, who have shown that the material which has been produced at high temperatures has the specific gravity and other physical properties of the mineral anhydrite.

It was formerly supposed that plaster prepared by baking at a temperature above 300 degrees loses completely its power of setting. Later observations, however, as those of Landrin, negative this view. Between 300 degrees and 400 degrees Landrin obtained plasters setting almost instantaneously when mixed with a small amount of water. When the temperature employed approached 400 degrees, the set plaster was softer, but the setting still took place quickly. These observations appear to show that the change to anhydrite is a very gradual process at temperatures below a red heat.

Reference has been made to the differences in (1) time of setting of plaster and (2) in hardness of the resulting material. Both of these properties are affected by the mode of baking. The hardest material is frequently obtained from the quick-setting plasters, but for certain purposes this rapidity in setting is of great practical inconvenience. Thus the moulder in pottery work must have leisure to fill in every detail of a design often complicated and intricate before the material with which he is working becomes intractable. Thus for many of the more refined purposes to which plaster is applied, extreme hardness in the set plaster is of less vital importance than a convenient period of setting. On the other hand, plasters which set very slowly give as a rule too soft a material, as well as being inconvenient in use. Plasters which hit off the happy medium are alone suitable for the work of the potter. The finer varieties of plaster prepared especially for use in potteries are obtained by a treatment which differs in many respects from that described above for the commoner kinds. In the first place, the direct contact of fuel or even flame is avoided, since this reduces some of the sulphate to sulphide of calcium, the presence of which is in many respects objectionable. Secondly, it is necessary that there should be a better control over the temperature, since, as has been seen, if the heating be carried too far the plaster, if not partially dead burnt, will set too quickly for the particular purpose to which it is to be put.

The arrangement employed in France is known as the _four a boulanger_, or baker's furnace. The temperature attained in the furnace itself never exceeds low redness. The material preferred is the softer kind of the granular variety of gypsum. This is put in in pieces of about 21/2 inches in thickness. After the baking several lumps are broken up and examined to see that there are no shining crystalline particles, which would indicate that some of the gypsum had remained unchanged. Before use the plaster is ground very fine. This point is of considerable practical importance. The consistency attained should be such that the material may be rubbed between the finger and thumb without any feeling of grittiness. Should there be particles of a size to be characterized as "grit," these will after use appear at the surface of the mould, with the result that the mould will have to be abandoned long before it is really worn out, i.e., before the details have lost their sharpness.

It is manifestly of considerable practical importance to understand the conditions which determine the time of the setting up of plaster. According to Payen, the rapidity of setting, provided the plaster has dehydrated at a temperature sufficiently low, depends entirely on the structure of gypsum employed. Thus, according to him, the fibrous kinds gives a plaster setting almost instantaneously. The water, he says, penetrates the material freely, setting takes places almost simultaneously throughout the mass. The hydration of each particle is accompanied by an expansion, and under the conditions specified, this expansion being unresisted takes place to the maximum extent, with the result of leaving cavities between the crystals, and producing a set plaster of less coherence and density. On the other hand, where granular crystalline gypsum has been used, setting begins at the surface of each group of crystals before the water has penetrated to the interior; the hydration is in consequence more gradual, and resistance being offered to the expansion of the inner parts, a harder and denser material is obtained. That this expansion contains an element of truth is indicated by the practice of employing the granular crystalline variety for the preparation of moulding plaster. The explanation appears, however, to be inadequate in several respects, especially in view of the fact that plasters for moulding are reduced to a fine state of division before use. It seems as if this treatment must, in great part at any rate, break up the crystalline aggregates.

In order to discover a more satisfactory explanation, let us examine the results of the chemical analysis of plasters used in commerce. One is struck by the large percentage of water they usually contain. Thus, four samples of ordinary plaster analyzed by Landrin have an average of 90.17 per cent. of CaSO4 and 7.5 per cent. of water, while two samples of best plaster contained 89.8 per cent. of CaSO4 and 7.93 per cent. of water. These numbers do not add up to 100, the difference being due to silica and other impurities of the original gypsum, amounting altogether to about 3 per cent.

It might be suggested that the reason why these plasters set more slowly than completely dehydrated plaster is owing simply to the fact that they contain, apparently, some unaltered gypsum, which serves to _dilute_ the action. Were this so, a similar result, as far as time of setting is concerned, should be obtained with a plaster containing a corresponding quantity of dead-burnt material. This, however, is not found to be the case. The time of setting appears, then, to be connected in some special and peculiar manner with the retention of water by the burnt plaster.

The following explanation of this connection is offered, an explanation only tentative at present, owing to want of experimental data.

The following substances are known:

Gypsum, and set plaster, CaSO4 + 2 H2O, containing 20.93 per cent. of water.

Plaster completely burned at moderate temperature, CaSO4, probably amorphous.

Anhydrite and dead-burned plaster, CaSO4, crystalline.

Selenitic deposit from boilers, 2 CaSO4 + H2O, or CaSO4 + 1/2 H2O, containing 6.2 per cent. of water.

The circumstance that the hot calcium sulphate can crystallize with 1/4 its normal amount of water indicates that for this proportion of water it has a greater attraction than for the other 3/4. Having a similar bearing is the fact that when burned at lower temperatures, gypsum only loses the last portions of water with extreme slowness.

Now, if it be the case that anhydrous calcium sulphate has a greater attraction for the first half molecule of water, then the operation of hydration will proceed very rapidly at first, more slowly afterward. Many such cases are known, e.g., that of copper sulphate. Conversely, if only 3/4 of the water of hydration be expelled during the baking of gypsum, the material obtained should hydrate itself more slowly. For our present purpose it will be convenient to recalculate the numbers given by Landrin (_vide supra_) so as to make the calcium sulphate and water add up to 100. This treatment of the numbers gives a mean result for the six analyses of 7.68 per cent. of water, the amounts not varying by more than 1 per cent.

It will be seen that the dehydration has never passed the composition corresponding to 2 CaSO4 + H2O; indeed, the material approximates more nearly to the composition 3 CaSO4 + H2O. It appears probable, therefore, that in the successful preparation of plaster the whole, or nearly the whole, of the gypsum is changed, but that this change does not result in the production of CaSO4, or of a mixture of CaSO4 and CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate.

In the case of the analyses, given by Landrin, of fine plaster for potteries, the percentages of water (8.14 and 8.08) correspond closely to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per cent. of water.

Some surprise may have been excited by the fact that the well known method of revivifying hydrated calcium sulphate has recently formed the subject of a patent (Eng. pat., No. 15,406).

The method described in the specification consists in reducing the materials (waste moulds, etc.) to small lumps, and baking between the temperatures of 95° and 300°. It is mentioned that the whole of the water must not be expelled. This is no doubt correct, but it must be effected by regulating the _time_ of baking, since by prolonging the operation all the water of crystallization can be expelled far below 300°. To secure even baking the mass is kept stirred by mechanical stirrers, a necessary precaution, since the operation is to be carried out in an ordinary kiln. The process is stopped when a portion of the plaster is found to set in the required time, a method of regulation which will probably be found to work well in practice.--_Chem. Trade Jour._

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SPACING THE FRETS ON A BANJO NECK.

BY PROF. C.W. MACCORD.

The amateur performer on the banjo, if he be of a mechanical turn, is often tempted to exercise his skill by making an instrument for himself; and the temptation is the greater because he can confine himself to the essentials. The excellence of a banjo in respect to power and tone depends mainly upon the rim and the neck, that is, supposing the parchment head to be of proper quality; but then the preparation of the heads is a business of itself, and the amateur is no more expected to make the head than to make the strings. So again, all the minor accessories, such as pegs and tail pieces, brackets and bridges, are kept in stock for his benefit, and he may justly claim all the credit if his efforts in connection with the two principal parts first mentioned result in the production of a superior instrument. Among these ready-made items is a "fret wire" of peculiar section, furnished with a flange ready for insertion into fine saw cuts across the neck, which much facilitates his work.

Of course, the correctness of the notes depends entirely upon the accuracy with which the frets are spaced, and the accompanying diagram exhibits a convenient method of determining the spaces by graphic means.

It is to be understood that when the distance from the "nut," N, to the bridge, B, has been determined, the first fret is to be placed at 1/18 of that distance from the nut, the distance from the first to the second is to be 1/18 of the remainder, and so on. To determine these distances by computation, then, is a simple enough arithmetical exercise; but it is exceedingly tedious, since the denominators of the fractions involved increase with great rapidity; being successive powers of the comparatively large number 18, they soon become enormous.

In the large diagram, the distance, A C, on the horizontal line corresponds to the distance, N B, on the instrument. At A erect a vertical line, and mark upon it a point B such that B C shall be exactly eighteen times any convenient unit, B I. In the illustration B C is 26 inches, and B I is 11/2 inches, so that B C is 27 inches in length. About C as a center describe the arcs, B L, I K, and through I draw a vertical line, cutting B L in D; draw the radius D C, cutting the inner arc, I K, in J, through J draw another vertical, cutting B L in E, and so on.

In the triangles, A B C, 1 D C, 2 E C, we have B I = D J = E F = 1/18 of the hypotenuse in each case, therefore the bases, A C, 1 C, 2 C, are divided in the same proportion, as required, at the points 1, 2, 3. And we might extend the arcs, B L, I K, and repeat the above operation until all the frets were located. But should that be done, the diagram might become inconveniently large, and some of the intersections might not be reliably determined. In order to avoid this, the spacing of the outer arc may be stopped at any convenient division, as L. The vertical by which that point is determined cuts B C at B', and through B' a new arc, B' L', is described. Through the points in which this arc cuts the radial lines already drawn, a new series of verticals is passed, which will divide another portion of A C as required, and by repeating this process the spacing of the whole neck may be effected by a diagram of reasonable size.

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GLOVE MAKING.

Glove making is almost a century old in this country, having been begun in the neighborhood of Gloversville and Johnstown, N.Y., about 1803. Until 1862 the manufacture of gloves in Fulton County, although even then the chief manufacturing industry, was of comparatively small importance. Gloversville and Johnstown were then quiet villages of from three to four thousand people. The flourishing establishments of to-day, or such of them as then existed, were small and comparatively unimportant. In 1862 the stimulating influence of a high protective tariff showed itself in the increased business at Gloversville, Johnstown, and the adjoining hamlet, Kingsboro. These became at once the leading sources of supply for the home market gloves of a medium grade. The quality of the product has steadily improved, and the variety has been increased, until now American-made gloves are steadily driving out the foreign gloves. The skill of American glovers is equal to that of foreign glove makers, and in some respects--notably in the quality of the stitching, and, in some grades, the shape--the American gloves are the best. Foreign expert workmen have been drawn over here from the great glove centers of Europe, so that the greatest skill has been secured here. The annual value of the glove industry in Fulton County has reached about $7,000,000.

One hundred and seventy-five glove makers and 20,000 people in Fulton County draw their subsistence directly from glove making. Some of the firms have a business reaching from $100,000 to $500,000 yearly. The majority, however, have small shops, and do a small but profitable business. Most of the work in Fulton County, as abroad, is done at the homes of the workers. The streets of Gloversville and Johnstown are lined with pretty and tasteful homes, in which the hum of the sewing machine is constantly heard during the working hours of the day, but the workers are exceptionally fortunate in being able while earning good wages to enjoy all the comforts and surroundings of home, and in being practically their own masters and mistresses.

Before the leather can be cut and sewed into the handsome articles that are sold over the counters of the retail dry goods houses and furnishing goods stores as gloves, the skins from which they are made must be specially prepared. The two important points in this preparation are the removal of the albuminous portion of the skin and the retention and chemical changing of the gelatinous part, so that it shall become pliable, elastic, and resist decomposition.

There are various methods which produce these results, and they are technically known as tanning, alum dressing, oil dressing, and Indian dressing. Each method produces a leather distinctly different from that produced by any other. All the preliminary processes of these various methods are alike in principle, although they vary somewhat in detail. The object in all is to remove the hair from the hide, separate the fleshy and albuminous matter, and leave only the gelatinous, which alone is susceptible to the chemical action and can be transformed by it into leather.

When the skins are received in the factory they are thoroughly soaked to open out the texture and prepare them for the removal of the hair. Then the skins are placed in vats of lime water, where, for two or three weeks, the lime works into the flesh and albuminous matter, and loosens the hair. The skins having thus been properly softened, the dirty but picturesque operation of beaming for removing the hair ensues. Before each beamer, as the workman is called, is an inclined semi-cylindrical slab of wood covered with zinc. The skin is first spread upon this, and the broad, curved beam of the knife glides across it from end to end, scraping and removing all the loosened hair, the scarf skin, and the small portion of animal matter adhering to the skin.