Cornish boiler might be so placed that while XCV.* By P. J. DAVIES, H.M.A.S.P., &c. Aur pipe Fig.4384 Ꭱ D Hot Water B Feed Cistern under it, as shown at FLUE, Fig. 122, and well up the back, as shown at K, Fig. 440, at the top of which must be fixed a damper, to prevent the water boiling away when there is too much fire for the amount of work to be done. In this engraving, Fig. 440, may be seen a method of WELDEL A KITCHEN BOILERS 69 B 78 C F 92 D be fitted with forty or fifty zinc plates. Further, he states that the zinc ball, with its perfect contact, generates a current of greater intensity than zinc plates; that a portion of the water is slowly decomposed, and the hydrogen evolved at the negative pole forces off the scale in thin flakes as soon as it is thick enough to become impervious. In this way, it is said, the scale is kept in loose thin flakes, which fall off instead of adhering to the boiler-plates. What is found useful in the case of marine boilers will also prove valuable in land boilers, which, though not so liable to internal corrosion, rapidly scale, owing mainly to the "penny wise and pound PRACTICAL NOTES ON PLUMBING.foolish" policy which admits of the use of almost any water in a steam boiler. By means of Mr. Hannay's "electrogens," it would appear that stationary boilers can be kept practically free from scale, the salts of lime and magnesia being deposited on the bottom as sediment, which can be removed by the mud-drum or blown off by the cock. In all feed-waters not naturally charged with a sufficient quantity of salt, Mr. Hannay recommends the addition of 4 parts to the 1000, a proportion which is necessary to make the "electrogen" active. The idea of putting steam boilers under the influence of a galvanic current is not new; but the method devised by Mr. Hannay appears to be original in its details, and has certainly been developed by elaborate experiment. In the course of the discussion which followed the reading of the paper, Mr. E. B. Martin and others gave advice which has been frequently emphasised in these columns-viz., that it is better to spend a great deal in doing something to purify the feed water itself than to do anything with it after it is in the boiler. Mr. Martin thinks there is enough in the "electrogen" to make it well worth while to try its application to land boilers, but he believes in "fresh water as the best of remedies, meaning, we take it, water from which the substances that produce scale have been eliminated previous to its admission into the feed tank. Mr. L. Perkins uses distilled water as pure as he can get it, and excludes grease and everything else likely to injure his boilers, some of which, he said, had been at work for 50 years, and remained just as good as they were on the day they were made. Mr. H. Maudslay also drew attention to the importance of purifying the feed water, although indirectly, by mentioning that a locomotive running over long distances would obtain water of qualities so different that there was room for another paper showing the variety of conditions as regards marine and locomotive boilers compared with stationary boilers, the feed water for which could be analysed and treated in a definite manner. Much of the discussion was not to the purpose; but Mr. Maginnis was the only speaker who had anything to say against the "electrogen," which he said he had tried some time ago, and the results, so far as he had been able to observe them, were not satisfactory. The vessels had, however, passed from under his superintend- your circulating tank should hold at least 50 ence, and all he appeared to be able to state gallons, the boiler should be of a suitable size, say was that he had been told that no throwing that it is a boot boiler, as at A., Fig. 439, size, off of scale could be seen. To his own say, 12in. from side to side, 8in. back, and boot, knowledge, however, zinc had been used for say, 4in. from the sole to the instep at 2. The years before the Admiralty thought of it. of the boiler. Let this boiler be about 15in. to width of the boot is generally the same as that The chairman, Mr. E. A. Cowper, thought 18in. deep. But it may happen that you have a that the "electrogen" was at least a pro- boiler with the range, so that you will not get mising invention, and referred to the mis- the chance of choice, but when you have the taken notion that it was small particles of choice see that you select one large enough, and copper carried over from the condenser should you not get the chance of selecwhich caused pitting; he thought it was tion, and have one which you think will quite as well to fix gauge-cocks, as at T, R, so proved beyond doubt that the pitting was not do the work, make your ideas known in the caused by the grease and oil becoming con- proper quarter, so that the onus of blame will verted into acid by prolonged boiling, and not rest with you through not speaking in time. Isaid that there was sufficient lime in sea- I again say that you should make it your duty to water to neutralise it. Referring to the see that it is large enough, and properly set, and strains often set up in badly arranged in such a manner that you can get a good fire flue boilers, Mr. Cowper said that a single-flued trates the connections and pipes between the round circulating tank A, and the boiler H. LK is the flowpipe, which takes the hot water from the top of boiler to the circulating cistern or tank A. BC is the return pipe which takes the colder water from the circulating tank to the boiler; C the emptying cock, the plug of which should be square-headed for a spanner; P the air pipe, which also answers as the draw-off pipe (see DRAW OFF; D E the cold water supply or feed (To be continued.) flue room and heating surface, for such boilers as a intermediate spot being instantly checked, a on Plumbing," p. 529, last vol., read as follows:-It would of 20in. of oil. PATTERN MAKING.-XXII. On the Use of Cores and Drawbacks. I have on many occasions set my boilers to heat too quickly. When such is the case the boiling hot water when entering the cold water will bump and rattle away like so many guns. This is easily got over by reducing the firespace with fire bricks, lumps, &c., &c. It is a good fault THE pattern maker is often in doubt as to the best method of taking out the recessed when properly understood. Some 11 years ago portions of a casting. Cores, drawbacks, dowelled I personally fixed one for Mr. Carey, the well- and wired pieces are the usual means resorted to known cheese merchant, at his residence at for the purpose. Sometimes all three methods Shepton Mallet, which heated 108 gallons of water are practicable, sometimes only two; rare are the in 15 minutes. The circulating pipes were only about circumstances in which the workman is reduced 8ft. long, which, when the fire was first lighted to one plan without having the option of made such a noise, although there was not the another. Broadly it may be said that castleast danger, that the servants all ran away outings can be made from patterns which are of the house, and it would also alarm many exactly like themselves. But in very many other people. I, in this case, took the flow pipe instances it would be very unpractical, unwise, and to the top of the hot-water tank, which allowed expensive to make them thus. Because a moulder the whole thing to work quietly, because then the can make drawbacks ad infinitum, and make joint hot water mixed with the hot water in the tank, over joint, as many as it is possible to put in the and not with the cold at the bottom. Such may pattern, that is no reason why a vast amount of be made to work quietly by reducing the water unnecessary labour should be undertaken which passage through the return pipe; the same may could have been avoided by a little judicious be checked by the use of elbows, long lengths of coring out. pipes such as a coil, &c. Before proceeding further, I wish to say a few words respecting the bumble or crackling noise which is occasioned by the sudden introduction of steam into water, and known as the "water hammer." Now, when steam is passing through a pipe, and suddenly discharges itself into cold water, the first contact of the two fluids is accompanied by a sudden condensation, and the rapidity of the impingement causes a sharp blow to be struck, usually at the point of entrance; B E Fig.444. F while, if steam be continued, a succession of such Yet we need not fall, on the other hand, into the error of taking out everything with cores, in order to lessen the difficulty of moulding. For cores are expensive, requiring making and drying, and the core boxes for a piece of work will very often entail more labour in the pattern shop than the pattern itself. Looking at the drawing of an intricate piece of work for the first time, it is not easy to arrange everything in detail, and to foresee every contingency that may arise, before deciding how best to make the pattern. Yet there is a rough-and-ready kind of intuition acquired by experience, and it is marvellous how readily a skilled hand will unravel the difficulties of a fresh job, and decide in his mind how he will set about it. Taking the various methods in succession-loose pieces, cores, drawbacks-he soon perceives which is is feasible and which is impracticable; then, what is easy and what is not so easy; then, lastly, which is good and which is best of all. So that before a tyro has grasped the leading features of the drawing, he has already settled in his own mind the lines on which he intends to work. But this almost unerring kind of intuition only comes after many years' habitual observation; and, also let us add, after many dearly purchased errors, for here, as elsewhere, failure is the stepping stone to success. To apprentices and young hands, there is nothing which presents such difficulty as this of the best method of moulding; and, in point of fact, it is seldom left to their choice. When the foreman gives out a job, he will almost invariably, except in very simple work, give directions as to the way in which it is to mould, and even then a watchful supervision is necessary with the less experienced hands. It is to help such as these in the matter of moulding that this article is written. The simplest way, perhaps, will be to illustrate our meaning as we go along by examples taken from actual castings. Take, for instance, a double bracket (Fig 142), such as is used for carrying the guide wheels of a gasholder. Here is a casting whose pattern might be made to mould with the edge A in the bottom, and the edge B in the top. Certainly the fillets or flanges round the edges would not allow of the pattern being drawn; but the sand forming the entire sides, including their bounding edges, could be drawn away on plates or "drawbacks." after which the plates are brought into position Well, we could make the pattern of the guide In case some of my readers should marvel respecting the meaning of this curiously applied term, let me explain. Fig. 143 represents two drawback plates laid against a pattern. On one side of the pattern is a long boss with ribs; on But it might be made to mould on its flat, just the other a thick bearing with its bracket. We as it is represented in Fig. 142, plan; in which suppose, as often happens, that there is not case the middle space, Fig. 142, back elevation, central space sufficient to draw these projecting would have to be taken away on a grid, of which pieces into, that cores would mean more work Fig. 144 gives a sketch. One side would then than the job is worth, and that the sides are be left loosely dowelled to come away in the top too deep to allow of jointing down from the top. box, and temporary distance blocks would Then we ram the sand around these particular maintain the sides at their proper distance apart sections of the pattern upon these plates, dividing during ramming. Yet neither would this be a the sand on each plate from that on the other wise course to adopt in a pattern of so flimsy conand from the outer sand by joints of brown struction as this, though it would be well suited paper or of parting sand, and afterwards to a stronger piece of work. In this case no two "draw back" the plates, with their complement castings would come out alike, but the sides of sand, from the pattern. The pattern is then would be winding and not parallel with each lifted out, and the face of the mould cleaned, other. We should, therefore, unhesitatingly Fig. 153. vote for a central core (indicated by the dotted line). Then our double bracket would mould on its side, as in Fig. 142 plan; the core-box being plain (Fig. 145), would involve little labour, and the distance between the frames would be perfectly uniform. The facing bosses, a a, will either be put one on a bottom board, the other on a cross-bar dowelled on top edge, or be measured into place while the core is being made. Of course the top frame would be dowelled upon the print. Take now one type of girder used at the ends of the gantries in overhead travelling cranes (Fig. 146), and which carries the flanged running wheels. Should we take out the centre in this case with a core? The frame in this instance also is double, with a central space; and here analogy would lead one at first sight to think it advisable to mould it on its side, making A the bottom and B the top, and to take out the central space with a core. But no; here we will use neither cores nor drawbacks, for although we cannot leave the bottom flanges loose round the edges, for the simple reason that their width could not be drawn inwards, we can leave the entire flange, made as a frame, loosely dowelled to the bottom edges, to be parted from the rest ENGLISH MECHANIC AND WORLD OF SCIENCE: No. 989. of the pattern by a sand joint. So our pattern will be exactly like the casting in this case, and the trouble of making a plain sand joint will be far less than that of making prints, and a corebox and preparing the cores besides. Fig. shows the pattern in sand completely rammed up. 147 Take another form of travelling girder (Fig. 148), where, instead of broad flanges, we have thin fillets, and bearings standing out beyond the fillets. Here we should leave bottom fillets and bearings loosely wired on, both inside and outside. Our sides are, say, 1in. thick, and our fillets lin. These latter would draw into the space left by the pattern after its withdrawal; but if, as we will suppose, the bearing blocks stand out an inch beyond these, they clearly will not come up through 1in. space. But we can get over the difficulty of increased thickness in one of two ways here. We could put the blocks on, lin. thick, outside the fillets, but not screwed, and draw them after the fillets; or we can take away the middle sand the entire foot with its bracketings, except, of Then, further, we must either core out underneath the steamchest flange, or leave it loose. A core would be far too troublesome, so by leaving that flange loose we can joint underneath it, and draw the half pattern from one box and the flange from the other. And, of course, the steam passages must be taken out by cores, the prints for which will be placed on the flange. The more completely to illustrate this chapter on the diversified forms of patterns, let us diswhich a methods in different cuss the lathe bed might be made. Everyone knows the familiar form of a lathe bed, with V-strips for the saddle of a slide-rest. The V-strips are planed all over, and that settles at once their position in the mouid. They will be in the bottom to secure the advantage of the soundest and cleanest metal. But we require to settle something beside the fact that the strips are to be cast downwards before we commence the pattern; and that is, how to taper the sides. Looking at the imaginary mould in section, Fig. 153, we see clearly that, though the sides of the bed can be made to withdraw readily from the sand, it is impossible to lift out the inner and outer bottom strips through the space left by the sides. Hence, our idea would be to remove some portion of this overlying sand, and so render the strips accessible. From this point of view the pattern could be constructed in three different ways, in order to provide for three methods of moulding, and the ready removal of the same in each case. The three diagrammatic views, much exaggerated, will clearly illustrate these three methods. In the first instance (Fig. 154) we plane no taper in the stuff which forms the sides, but give, instead, in. to in. of taper (according to the depth of bed) to the ends of the bridges or cross bars, so that the bed shall spread bodily in section. Here the sand forming the inside of the bed will be lifted away on plates, after the removal of the top box, and the pattern itself will be drawn immediately afterwards. The fillets, a a, would have to be wired on to lift with the inner sand and the outer Vee's: also bb to be drawn into the middle space left by the lifting out of that sand. The process adopted in method two (Fig. 155), would be the same in principle as the first, but the modus operandi would be exactly the reverse. Here the outer sand would be lifted away, either on drawbacks or on an outer encircling plate; then the pattern would be drawn leaving the In the third method (Fig. 156) the middle sand remaining. In this case the inner strips, cc, and the outer fillets, a a, would be left loose. medial lines of the sides are parallel, and taper is given to their inner and outer faces. The. bottom strips, both inside and outside, are wired on, and the screws which unite the sides to the crossbars are withdrawn one by one as the ramming goes on, and when all is rammed up and the top lifted away, the pattern is drawn piece by piece, leaving the sand behind. Then either middle or outside is taken away on plates, and the loose strips drawn in. Fig. 157 shows an encircling plate as used for lifting the outside of a mould away bodily. The last method is to take out the inside with Only the outer pieces, a, a, cores (Fig. 158). It makes a cleaner and truer The core-box may be made the casting, and as the cores are very plain, the expense is not great. are wired on. entire length of the pattern, and the bridges put in, as into the pattern, or short core-boxes may be made to reach from bridge to bridge. 66 12° 5' 34-6' 'Trigonometrical Calculation of the Path of the Ray through two Leases. (A) = 4° 11' 23-7" d "Let 1 Sin. == sin. a. (A) = μ .(1) .(2) (3) Some of my readers may be disposed to put the be the index of refraction of the first, question, Why lay so much stress on matters of and u' that of the second lens; the four points of moulding, instead of describing simply how such intersection of the directions of the ray with the and such patterns are made? I reply, that in axis A, B, A', B', and the four angles are found any save the simplest jobs, scarcely any two by the following formule, in which a is the first men would go to work in precisely the same way. A pattern maker must understand mould-angle of incidence, and b, c, d, a', b', c', d, are ing well, and even here the usages of shops 1st Refractionauxiliary angles. differ. The bare description of one pattern would be of little assistance in the construction of another, having perhaps a general resemblance to that, yet differing from it (from the moulder's point of view) in some very important particular. I know of no trade where the methods of working differ so widely as in pattern-making. Many patterns can be made to mould in three or four different ways, and the workmen, therefore, instead of working in certain set grooves like those of many handicrafts, which will occur to the mind, must devote to each new job some amount of originality of thought, and modification of construction. It is for this reason that piecework is so difficult of adoption in the pattern shop. (4) (5) (6) 2nd RefractionSin. = A+ rat 72 Sin. du sin. c... (B) = (A) + c B = 2 = (5 sin. d d... sin. (B)-1) 3rd Refraction Sin. a' = (7) (B) sin. (B) ... (8) Sin, b' = sin. a'... 1 μ (A') = (B) + b' A' = 13 ( .(9) ...(10) ...(11) N all the preceding formulæ for combining the lenses of an achromatic objective, the expressions used to determine the aberrations are only approximately correct. The greater the 4th Refractionproportion between focal length and aperture, the less will appear the discrepancy, between the theoretic and the actual aberration. We come now to a method, in which the author calculates the paths of the rays, according to the established law of refraction; and by this means, he is enabled to test the accuracy of any of the approximate methods, one of which must still be used to find a ratio sufficiently near that finally selected. = Sin. d'u' sin. cʻ (B') = (A') + c'—ď′ §3. If in these expressions we suppose the angle of incidence a to be very small, some easy reductions of the preceding formula will lead to the following: The first section of Littrow's article is devoted to an explanation of his way of deriving the two fundamental equations, which Sir John Herschel has used in his theory of the object glass. With through two lenses, on the supposition that the point Optical Calculation of the Path of a Ray of Light this, we have not at present to deal, and my of incidence is infinitely near the axis: while in abstract must be confined to the chief formula2 this point might be assumed at any given and examples, together with the author's remarks on their use. I have altered his nomenclature in many places, hoping thereby to lessen the confusion to which a multiplicity of characters is liable. "§4. These two systems of equations are of the greatest use in the theory of telescopes, and may be readily applied to determine not only what is the best construction, but whether any proposed construction be good. The general requisite, After his remarks on Herschel's theory, Lit- which every really good telescope must possess, trow thus proceeds :-"If the spherical aberra is, that for every proper value of the angle of tion of the rays, as well as the chromatic aberra-incidence a, and for the values of μ and cortion be required to be rigorously destroyed, not responding to the red and violet rays, the distance only for rays incident very near to the axis, but of the last point of intersection B' in § 2 and § 3 likewise for those which are incident at some be nearly the same: because, if the value of B' notable distance from it, it becomes necessary to varies greatly for different angles of incidence, calculate the path of the ray through both and for rays of various colours, the image can lenses, not only by the common approximate never be distinct." optical formulæ, but likewise accurately by trigonometrical formula, and to take into account the thickness and distance of the lenses. Let, therefore ri, denote the radii of the first lens of crown glass, rs, r1 those of the second of flint glass (where the radii are to be taken negative for concave surfaces); call t the thickness of the first lens, t' that of the second, and s the distance of the second refracting surface from the third. Let the ray parallel to the axis form with the perpendicular on the first refracting surface an Crown-index = 1·53; dμ = .01; flint index angle of incidence = a, and let us suppose 1·60; dμ' : = "04: ἀμ | αμ' 25. Thickness that the directions of this ray after the of crown lens 01 t. That of the flint t' and first, second, third, and fourth refractions, in- the separation s are assumed = o. The angle of tersect the axis in points whose distances from incidence a = 12°, and the focus of the crown the 1st, 2nd, 3rd, and 4th refracting surfaces are lens is the unit of dimensions of the objectrespectively A, B, A' B', and that the angles glasswhich these rays form with the axis at these points are respectively (A), (B), (A'), (B'). It will now be easy to draw the necessary figure, and to deduce the expressions which follow. Next compute B' by the equations in § 3, both for the red and violet, as well as the mean rayMean ray μ = 1.53, μ' 1.60. B' 1-390782. Red ray μ 1-52, μ' 1·56. B' 1.390817. Violet ray μ = 1.54, μ' 1.64. B' 1.390819. Comparing these with the former value, it appears that while the central chromatic error is well eradicated, the spheric error is much undercorrect, as might have been inferred from the arrangement of the curves. The problem to examine any proposed double object-glass in all its relations is, therefore, perfectly solved by the two systems of equations in § 2 and § 3. § 4. But it is a very different question to determine for any given values of μ, μ', and du/du', a double object-glass which shall have all the properties required in practice. The principle of these is, that all rays, red as well as violet, incident near to, as well as far from the axis, shall all unite after the fourth refraction in one point of the axis as far as possible; because the spherical aberration cannot possibly be remedied, for all rays without exception, with mathematical strictness. For this All writers on optics have hitherto followed different methods for solving this problem in a manner sufficient for practical purposes. The general custom has been to destroy the chromatic aberration for rays incident near the axis only, and the spherical aberrations for the extreme rays incident near the border; and then the solution of our problem requires the fulfilment of two conditions only, which may be effected by taking proper values for two of the four radii ri, r2, 73, 7, while the other two radii remain indeterminate. In other words, if it be only required that the mean rays incident near to and far from the axis, and besides these coloured rays only which are incident near the axis shall unite after the fourth refraction in one and the same point of the axis, then it is possible to find for every radii r, r, a flint lens that shall fulfil the two crown lens, whatever may be the ratio of the conditions proposed, and the problem is, therefore, properly an indeterminate one. reason, very different assumptions have been hitherto made for the ratio of the two radii, in order to insure to the telescope some other valuable property-for example, a greater aperture of the object glass, or a shorter focal length, or even to render the complicated calculations, which are otherwise required, more commodious. Thus, Klügel has considered it as advantageous that the refractions of the ray-i.e., the quantities (a - b), (d — c), (b′ — a'), (ed), should be as smali as possible; for which purpose he assumed the ratio of the radii r, and 2 nearly as 1 to 3. Professor Bohnenberger believed, on the contrary, that the ratio 2 to 3 deserved the preference, because this ratio would better correct the two aberrations. Euler, again, was for making the spherical aberration of the first lens a minimum, and therefore adopted the ratio of 1 to 7. Klügel's purposed, in his Analytical Dioptrics, to give the telescope the greatest possible aperture, which forced him to assume the two radii equal." After some remarks on Klugel's method, Littrow says:-"It appears to me that the problem may be solved in a manner more convenient and more safe than has hitherto been done. For this purpose we will, in the first place, take the equations of § 3, and eliminate from them the quantities A, B, and A'. The interval between the second and third refracting surfaces & being extremely small in proportion to the focal length, we will put = o, and, besides, neglect the second and higher powers of the thicknesses of the two lenses t and t. On this supposition, the elimination gives : |