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To Repair Old Masonry How is old masonry to be repaired when to be connected with new?-C. R. D.

The mortar of the old should be thoroughly cleaned off wherever it is injured along the surface where the junction is effected, and the surface thoroughly wet. The bond and other arrangements will depend upon the circumstances of the case; the surfaces connected should be fitted as accurately as practicable, so that by using but little. mortar, no disunion may take place from settling.

An expedient, very fertile in its applications to hydraulic constructions, has been for some years in use among the French engineers, for stopping leaks in walls and renewing the beds of foundations which ha ve yielded, or have been otherwise removed by the action of the water. It consists in injecting hydraulic cement into the parts to be filled, through holes drilled. through the masonry, by means of a strong syringe. The instruments used for this purpose, as shown in the figure, are usually cylinders of wood or of cast iron; the bore uniform, except at the end, which is terminated with a nozzle of

the usual conical form; the piston is of wood and is driven down by a heavy mallet. In using the syringe, it is adjusted to the hole; the hydraulic cement in a semi-fluid state poured into it; a wad of tow, or a disc of leather being introduced on top, before inserting the piston. The cement is forced in by repeated blows on the piston.

Sand should be avoided in the mortar for injections, as it is quite likely to cause trouble by settling to the bottom of the syringe and preventing the formation of a homogeneous mass.

Why Boilers Explode

Can you give any reason for a boiler exploding shortly after being tested?-M. O. P.

There is good reason for believing that most of the mysterious explosions of boilers which stand the Inspector's test, and then explode at a much less pressure,

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METHOD OF RETAINING OLD MASONRY.

are due to the weakening effects of unequal expansions, for a boiler that will stand a hundred pounds test this week cannot explode next week at fifty pounds pressure, unless it has suddenly become wonderfully reduced in strength, and no corrosion or other natural cause, with which we are acquainted, save expansion, can produce this result. When we consider that strains from difference of expansion are generally greatest when firing up, and when there is no pressure in the boiler, we can see that the time may arrive when a crack is started or

the parts weakened, so as to give way under a moderate pressure just after the test has been made; and this is the probable reason why so many boilers explode in getting up steam, or so soon after, or upon pumping in cold water, or, even, as in a recent case in England, while cooling off.

A Portable Bookcase

Can you tell me how to make a bookcase that can be easily taken apart-without the use of nails or glue?-C. S. H.

Provide upon the ends of the shelves a tongue, or tongues, long enough to pass through the thickness of the sides and reaching to one inch beyond. Each side must be pierced with apertures to permit of this, and the tongues also are pierced, each one with a square hole. When the tongues at the ends of the shelves are pushed through the mortises in the sides, all that requires to be done to make a firm structure is to insert pegs in the holes in the tongues, every part being thus interlocked immovably. The general appearance of the set of shelves is not impaired by the arrangement just described indeed, it is suggestive of a very neat and compact piece of work,

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The Wright demand, or discount, meter is used in connection with a watthour meter. It registers the maximum amount of current used on the line in which it is installed. The theory of its operation is as follows.

A sealed glass tube of the shape shown in the figure is partly filled with a suitable liquid "L." Behind the tube "s" is placed a scale; "A" is an air chamber, "H" a bulb, about which is wound a coil of wire of such resistance that an increase in current strength will cause a large rise in temperature. The heat, due to the current passing, causes the air to expand in bulb "H" and forces the liquid up in the right tube. If the temperature is high enough it will force. some of the liquid over into the tube "S." The greater the current passing, the greater the temperature rise and the larger the quantity of liquid forced into tube "S." Therefore the liquid in tube "S" is a measure of the maximum current strength used. The scale behind "S" serves as a convenient indicator of the current registered in tube "S."

The customers bill is figured as follows.

Suppose for the first thirty hours the maximum amount of current is charged for at the rate of fifteen cents a kilowatthour and the remaining amount at ten cents a kilowatt-hour. For the same number of kilowatt-hours a bill will be less, the smaller the maximum amount of

current used. This is an inducement for the customer to use a few lights or small power for a long time, rather than a large amount for a short time.

In electric power plants extra apparatus is often needed to take care of the large load which is thrown on the station at certain times of the day. This apparatus is in operation, usually, for only a few hours a day and for the remaining time is idle, not earning money, while the interest on the money invested in it continues.

By the use of a discount meter customers, whose use of current cause this apparatus to be installed are made to pay more, according to the amount of current they consume from their appa

ratus.

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Lathe Dogs

What are lathe dogs?-R. H. H.

As the frictional contact of the work on the live center is not sufficient to turn it, some device must be used to make the work turn with the center. To accomplish this a lathe dog is used. For round work, such as shafting, a dog like that shown in Fig. 1 is often used. The shaft or piece to be turned is placed in the hole A, and held firmly in place by the set screw B. The tail-piece C is put through a hole in the face plate and the work rotates with the live center.

While this type of dog is satisfactory in most cases, the fact that the contact between the dog and the face plate is beyond the end of the piece, introduces a bending strain which is appreciable in slender work. To avoid this, dogs are made with a straight tail, and driven by a stud projecting from the face plate.

LATHE DOGS.

SOLENOID.

Suppose two boys, Ned and Sam, to face each other, holding a stout spiral Each boy atspring between them. tempts to revolve this solenoid about its axis, clockwise. The first effect is that the spring will absorb some of the energy, expended by the two boys, in producing a torsion, stress, or strain in the spring. If their exertions be equal a state of equilibrium will soon be reached and the solenoid will remain at rest. If Ned exerts the greater strength the solenoid will revolve anti-clockwise, as appearing to Sam, and energy will be transmitted along the axis of the solenoid from right to left. Now when Ned gets tired and reduces his strength until the coil ceases to revolve, and then Sam "lets go," his end of the coil will tend to continue the rotation in the same direction, and, in so doing, gives up the energy stored as torsion or strain. If Sam is a little careless he may get a jab in the hand, or a bad "kick.' Just so a wire, carrying a current, has a certain amount of energy stored in its surrounding field, which, at the moment when the circuit is opened, is given back as an "extra current." Its strength will depend upon the conditions; if a highly charged and large electro-magnet be included in the magnetic field of the circuit, the effect of the extra current may be highly destructive.

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hangers as nearly as possible in a horizontal line, the shafting should be placed in the boxes and attached to the hangers. For lining the shaft, a level and a fine grass or silk line are indispensable. The line is tightly drawn, horizontally, a short distance from the position it is desired that the shaft shall occupy, and the distance from the surface of the shaft to the line is measured and made equal near each hanger by a stick such as is shown in Fig. 202.

The level is used to make the shaft horizontal, and, if the hangers are adjustable in two planes, the operation is quite rapid.

When other shafting is to be erected parallel to the first, if the distance does not exceed twelve or fifteen feet, a long stick such as shown in Fig. 1 may be used by driving a nail into the end of the stick to allow some adjustment. The level is used as before.

When the distance is great, or obstacles prevent the use of the stick as suggested, a line may be drawn on the floor of the shop by dropping a plumb line from near the ends of the first shaft and connecting the points located. Another line, directly under the desired location, may be drawn by direct measurement, and the second shaft erected by dropping a plumb line to this second floor line near the ends of the second shaft. This method may be employed, with such variations as the case may demand, even though a floor or wall be between the locations.

In leveling up long lines, or around machines, or through walls, the hydrostatic level is a most convenient tool. It consists of two graduated glass tubes set

in suitable bases and connected by a rubber tube. When the rubber tube is filled with water, and the glass tubes placed vertically on the shaft, the fluid should stand at the same gradation in each glass. These levels are made with self-acting valves to prevent the escape of the fluid.

When pulleys or hangers make the direct application of a level to the shaft. impracticable, leveling hooks, in connection with a wooden straight-edge, as shown in Fig. 1, are very convenient. These may be made of wood or metal, and of lengths suitable to the case in hand.

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Joining Leather to Iron Question: How can leather be glued to iron?-B. E. S.

Answer: To glue leather to iron, paint the iron with some kind of lead colorsay white lead and lampblack. When dry, cover with a cement made as follows: Take the best glue procurable; soak it in cold water till soft; then dissolve in vinegar with a moderate heat, and add one-third of its bulk of white pine turpentine. Thoroughly mix, and by means of vinegar make it the proper consistency to be spread with a brush. Apply the cement while hot; draw the leather on or around quickly; and press tightly in place. In case of a pulley, draw the leather around tightly as possible; lay and clamp.

Self-Hardening Steel

What is self-hardening steel and for what is it used?-S. B. T.

Hardening steel, as its name indicates, is almost self-hardening in nature, generally the only treatment that is required to harden the steel being to heat it red hot and allow it to cool. Sometimes the steel is cooled in an air blast or is dipped in oil. It is not necessary to "draw the temper." The self-hardening quality of steel is given to it by the addition of Chromium, Molybdenyum, Tungsten, or one of that group of elements, in addition to the carbon which ordinary tool steel contains. Self-hardening steel is comparatively expensive, costing from 40 cents and upwards per pound, some of the more expensive grades costing $1.00 or so. When in use, self-hardening steel will stand a much higher cutting speed than the ordinary so-called carbon steel. For this reason it is much more economical to use, although its first cost

SELF-HARDENING STEEL TOOL.

is higher. Self-hardening steel cannot be cut with a cold chisel and must be either cut hot or nicked with an emery wheel and snapped off. Great care must be used in forging it, as the range of temperature through which it may be

forged is comparatively slight, running from a good red heat to a yellow heat. Some grades of self-hardening steel may be annealed by heating the steel to a high heat in the center of a good fire and allowing the fire and the steel to cool off together. Steel which has been annealed in this way may be hardened by heating to the hardening heat and cooling in oil.

Self-hardening steel is used to a large extent in modern practice for lathe tools, much being used in the shape of small square steel tools held in special holders. Such a tool is shown in the accompanying figure.

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The usual way to connect a powerfactor meter to a three-phase circuit is shown in the diagram. A, B and C are the three phases of the three-phase circuit. Only two series transformers, a and c, are necessary. The shunt transformer may or may not be necessary, according to the potential between A and B. Leads c, d and e are connected to what is virtually a two-phase induction motor, in the interior of the powerfactor meter. It is this motor that, partially counter-balanced by the shunt coil in the interior of the meter, gives torque to the dial pointer.

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