be largely removed by boiling the finished articles in a solution of caustic soda. Both the air bath and steam bath are in use for heating; the latter, at present, in the majority of cases. This latter heating, which effects the change in the rubber, is frequently called the "curing" of the rubber. In the manufacture of hard rubber articles, the East Indian, and especially the Java and Borneo caoutchouc is used, Para rubber being too expensive, besides being not so well adapted. While in the manufacture of soft rubber the burning or curing was the last process following the shaping of the articles, in the manufacture of hard rubber the curing is generally done before the article is finally shaped. Gutta percha, balata and colophony resin are often added to modify the hardness and elasticity, while a large number of mineral substances, such as chalk, gypsum, zinc, oxide, asphalt, etc, are added chiefly for cheapening purposes

(20) Will you please give me the practice of surveyors in allowing for the diurnal variation of the compass needle on the vernier? I notice some days the needle will vary nearly a quarter of a degree between morning and noon of the same day and other days; if the weather is cool and cloudy, not nearly so much. Is there any rule for determining the amount for each month throughout the year, and at any time of the day? J. B, Danville, Ill. ANS.-We think the general practice is to neglect the diurnal variations of the needle entirely. There is no rule for determining

the amount of this variation, but the values given in the following table, which are from Johnson's "Theory and Practice of Surveying," probably represent average values of this variation for latitudes between about 35° and 45° north, and may be applied as corrections for the diurnal variation. These values are correct to the nearest minute for Philadelphia, where the observations were made, and are close enough for the purpose. Since the diurnal variation is probably due to the influence of the sun, it is greater in summer than in

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books giving information on the preparation of oxygen and ozone?

W. G., Huntley, Ill. ANS.-(a) Compressed oxygen, and probably ozone, may be purchased from Underwood & Co., New York. (b) The preparation of oxygen and ozone is described in any book on inorganic chemistry. We would recommend you "Inorganic Chemistry," by Ira Remsen, American Science CourseAdvanced Series. This book may be obtained through the Technical Supply Co., Scranton, Pa.

(23) How can I obtain the shape of the slats for the louvre windows shown in the accompanying sketch?

H. C. L., Stanwick, N. J. ANS. The semicircular window frame may be considered as a half clyinder, and the louvres as oblique planes passing through it. In Fig. 1, A shows the elevation of the half cylinder; B, a section on the line kl; C, a side elevation in which so represents the slant of the louvre; and D, a development of the section on the line so in C. This development is obtained as follows: Divide the semicircle t ku, in A, into any number of equal parts, as t 1, 12, 23, etc.,

drawn through the points t' 9 8 7 0 7 8 9 u' gives the shape of the oblique section through the half cylinder. We will now

consider the louvre a b c d in B. To obtain the shape of a b c d, we project the points a and c to m and n on the line so in C, giving the distances o m and on which are laid off from o on o 8, in D; aob will then be the shape of the upper surface of the louvre, and co d, the shape of the under side. The distances a b and c d, representing the surfaces of the louvre, might have been taken directly from B without projecting in this case, but in obtaining the development of the other louvres it would be necessary

to project, and consequently the projections were made to preserve a uniform method. To develop e f g h, we project these points to qvrp on 80, in C; then lay off the distances o v, op, o q, and or on os in D and draw lines parallel to t'u' through the points v, p, q, and r.

and from these points drop perpendiculars to the line tu. Since the figure is symmetrical with respect to the axes kl, we will consider only the right half in making the projec

tions to C. The next step is to project the points k, 4, 5, 6, u, in A, to the line so in C; 80 will then be the true length of k o' in the oblique section, and s 7, 88, 89 will be the true distances for 4' 4, 5' 5, 6' 6. In D we lay off t'u' equal to tu in A, and project the points 1, 2, 3, etc. to this line; these perpendiculars will then be the lines on which to measure off the true distances obtained in C. Lay off s'o equal to 8 o in C, and s' 7, 88, 8' 9, on the perpendiculars at each side of the center line equal to 8 7, 8 8, and 89, respectively, in C. A curved line

SCIENCE AND INDUSTRY. FIG. 1

e e' f'f will then be the shape of the upper surface and g g'h' h of the lower surface of the louvre ef gh. The other louvres should be developed in the same manner. In laying out the work, the board from which the louvre is to be cut should be beveled off on one edge to correspond with the angle a cd in B, as shown at E. The center lines should then be drawn, and the development of the surfaces of the louvre laid off on the upper and lower sides of the board. The ends of the board should be beveled off to conform to the curve b d

heating it. I have connected my heater to this tank as follows: I ran a pipe from top of heater to the side of an expansion tank. From the bottom of the expansion tank I ran a pipe and connected it with the opening a of the tank. From the opening b I ran another pipe and connected it to the bottom of the heater. The heater and the expansion tank become very hot but there is no circulation through the horseshoe heater. What is the trouble? E. S. A., Marrietta, Ohio.

ANS. Our correspondents submitted other sketches which are not clear enough for us to understand. We cannot therefore definitely state what is the trouble with his arrangement. However, Fig. 2 shows how

escape. This expansion tank allows for expansion of the water when it is heated and is necessary for such an apparatus. The arrows show the general direction of the circulation. The pipes are graded so that any air inside the apparatus can rise and escape from the expansion tank.

(25) (a) How can I take a picture with an ordinary hand camera to resemble the one here given? This is a clipping from a newspaper. (b) How can I proceed to solder iron, using borax as a flux? I have tried several times with no result. (c) What is the best book treating on character building?

S. T., Houston, Texas.

ANS. (a) A photograph of the person is first taken by the ordinary process. This photograph is then photographed upon a sensitized copper plate and the lines etched out, making what is called a half tone, and this half tone is used by the printers to reproduce the picture such as you submitted. Half tones are made by persons who make a business of their manufacture, and you would probably have difficulty in attempting to make one yourself. (b) See Home Study Magazine for December, 1898, question 514. Use borax which has been melted and broken to a coarse powder. (c) "Character," by O. S. Marden; published by the Success Co., New York, N. Y.

**

(26) What are the ingredients of solder such as is used for sealing condensed milk cans? L. K., Waterbury, Conn. ANS.-Solder such as is used for sealing condensed milk cans is made of equal parts of lead and tin. It is important that the ingredients used should be as pure as it is possible to obtain, and any dross that forms on the surface when they are melted should be removed.

(27) Please give me the title of a good book on concentrating machines.

H. C. J., Gold Hill, N. C. ANS.-You will find some practical information in a concise form on this subject in The Coal and Metal Miners' Pocket Book. This can be secured from the Technical Supply Co., Scranton, Pa. Price $3.00.

(28) Given a circle of known area A. Required the radius of a circle whose center is on the circumference of 4, and whose circumference where it intersects A will divide A into two parts.

G. A. N., New York City, N. Y. ANS.-See the Mechanic Arts Magazine for March, 1899, page 88, in which this problem is fully solved. Same can be had by remitting 10 cents to this office.

Vol. VII

ANY articles have appeared from

MAN

time to time in the engineering press in reference to the steam engine, its design, operation, and economy, but comparatively few have contained much information on the methods employed in a trial test. It is of great importance in power plants to know what amount of coal is used to produce a given amount of power. The electrical engineer should have as much interest in the amount of coal consumed as the steam engineer, for the more efficient the engine, the greater will be the electrical output. It is the purpose of this article, therefore, to give some of the practical points which are essential in the testing of a steam engine, and to show the ordinary way in which such a test is conducted.

At the outset we should have well fixed in mind the object of the test. There are usually four things sought after: (1) the amount of coal consumed per horsepower per hour; (2) the amount of dry steam used per horsepower per hour; (3) the number of heat units evolved from the fuel and used to generate the steam, and (4) the ways in which part of the heat is lost.

In some cases it is desirable to ascertain a fifth unknown quantity; namely, the mechanical loss due to friction; in other words, the friction horsepower. This is easily found, as will be shown

No. 2

later. The first four are more difficult problems, and will be treated at some length.

Before considering the first let us investigate for a moment what takes place in the boiler generating the steam, and the ways in which part of the heat units evolved from the coal are lost, and become unavailable for doing work. When combustion is perfect, one pound of pure carbon should yield about 14,500 B. T. U. (British) thermal unit), 1 B. T. U. being equivalent to equivalent to the amount of heat necessary to raise 1 pound of water 1° F. The mechanical equivalent of heat which was originally determined by Joule, and afterwards corrected by Prof. Rowland, is 778 foot-pounds of work, therefore, the theoretical number of units of work which should be obtained from 1 pound of pure carbon is

portion of it is lost through radiation, another portion by conduction, but a much greater quantity than either of these is lost in the engine cylinder by condensation of the steam itself. very important problem to the steam engineer is cylinder condensation, and until we shall be able to operate our steam engines by means of superheated steam we cannot hope to remedy this fault very materially.

The best simple engine is capable of using only about 8 per cent. of the total heat supplied by the coal. Α triple expansion engine will run up to 15 per cent. efficiency, which obviously shows the advantage of compounding, but even then we have 85 per cent. of the heat escaping, and beyond recovery for doing work.

In order to ascertain the amount of coal used per horsepower per hour, the amount of steam generated per pound of coal must be found, and in order to do this with any degree of accuracy a boiler test should be made, after which the engine should be indicated. As the present article deals only with the testing of the engine, it is out of place here to go into boiler testing. The coal value is only approximate at best, so that it is best to assume such a value as the boiler rating warrants, and accept corresponding results. Having these two values it is evident that the coal used per I. H. P. hour is very easily found.

It is necessary to know the condition of the steam in order to determine the losses, the greatest of which is condensation in the cylinder and piping. By the condition of steam we mean the quality of steam which is the percentage of dryness. This can be found only by a a careful test with a steam calorimetor, which will be described shortly. To find the percentage of condensation, the amount of dry steam

used per horsepower per hour, the steam used per hour, and the I. H. P. must be known.

The latter is of course computed directly from the indicator card. The amount of dry steam used per horsepower per hour is found by first determining the total amount of steam used per I. H. P. per hour and then subtracting the percentage of moisture in the steam as shown by the calorimeter.

The heat used (B. T. U.) per horsepower per hour is found by first ascertaining the amount of steam per horsepower per hour taken from the boiler, and then multiplying each pound of steam by the total heat value of one pound at the corresponding temperature as given in the steam table.

The thermal efficiency of a steam engine, may be expressed by the formula,

Before describing the test it may be well to explain the use of the apparatus employed. Most of us are more or less familiar with the steam engine indicator, and much depends upon the observation of the cards if the results are to be reliable. It is a well known fact that no two persons will observe the same thing in the same way. With regard to readings on scientific instruments, or even on a common draftsman's scale, the results of personal observation will vary. Therefore errors are found to creep into the results of a test such as the one that concerns us here.

On account of these errors of observation it may readily be seen that the