How to Use a Slide Rule Will you please explain how to use a sliderule?-C. K.

Holding the rule so that the figures are right side up, four graduated edges will be seen, of which only the upper two are used in the problem about to be described. The method of using the two lower scales would be exactly the same, the difference being, that they are twice as long, and that the slide is above instead of below the scale,

Move the slide to such a position that the graduations agree throughout the length of the scale, and place the runner at a division marked 1, and the rule is ready for use. Arrange the factors to be dealt with in the form of a fraction, with one more factor in the numerator than in the denominator, units being introduced if necessary to make up deficiencies in the factors.

Thus, to multiply 6 by 7 by 3 and divide by 8 times 2, arrange the factors as follows: 6x7x38x2. The factors in the numerator show the successive positions which the runner must take; those in the denominator the positions of the slide. Thus, to solve above example, start (1), with runner at 6 on the scale, always reading from the same side of runner; (2), bring figure 8 on slide to runner; (3), move runner to 7 on slide. The re

sult can now be read on the scale; (4), bring 2 on slide to runner; (5), move runner to 3 on slide. The result is read directly on the scale at position of run

Another example: Multiply 11 by 6 by 7 by 8, and divide by 31. In this case arrange the factors 11x6x7x8: 1x1x31. Start with runner at 11 on scale, move 1 on slide to runner, move runner to 6 on slide, move 1 on slide to runner, runner to 7 on slide, move 31 on slide to runner, runner to 8 on slide; read result on scale at runner.

The numbers on the slide-rule are to be considered significant figures, and to be used without regard to the decimal point. Thus the number on the rule for 8 is to be used as .8 or 80 or 800, as may be desired, even in the same problem. The significant figures in the result are readily determined by a rough computation. In case the slide projects so much beyond the scale, that the runner cannot be set at the required figure on the slide, bring the runner to 1 on the slide, then move the slide its full length, until the other 1 comes under the runner. Then proceed according to directions above; i.e., move runner to number on slide, and read results on the scale. 6x25x3.5x 7x7x31x426x914x1x1.

Begin with the first factor in the nu

merator, and multiply and divide alternately

x 6, +, x 25, ÷ 426, x 3.5, ÷ 914, etc., until all the factors have been used, checking them off as they are used, to guard against skipping any or using one twice. To multiply, move the runner, to divide, move the slide; in either case see that the runner points to a graduation on the slide corresponding to the factor. The result at the end, or at any stage of the process is given by the runner on the stationary scale. Or, to be more exact, the significant figures of the results are given, for in no case does the slide-rule show where to place the decimal point. If the decimal point cannot be located by inspection of the factors, make a rough cancellation.

"Vena Contracta""

Will you please explain the theory of the "vena contracta"?-T. J. W.

If we suppose the sides of a vessel containing water to be thin, and the orifice to be a small circle whose area is A, we might think that the quantity of water E discharged in a second would be given by the expression AV2gh, since each particle has, on the average, a velocity equal to V2gh, and particles issue from each point of the orifice. But this is by no means the case. This may be explained by reference to the figure, in which A B

represents an orifice in the bottom of a vessel-what is true in this case being equally true of an orifice in the side of the vessel. Every particle above A B endeavors to pass out of the vessel, and in so doing exerts a pressure on those near it. Those that issue near A and B exert pressures in the directions M M and N N; those near the center of the orifice in the direction R Q, those in the intermediate parts in the directions PQ, P Q. In consequence, the water within the space P Q P is unable to escape, and that which does escape, instead of assuming a cylindrical form, at first contracts, and takes the form of a truncated cone. It is found that the escaping jet continues to contract, until at a distance from the orifice about equal to the diameter of the orifice. This part of the jet is called the vena contracta. It is found that the area of its smallest section is about 58 or 0.625 of that of the orifice. Accordingly, the true value of the efflux per second is given approximately by the formula E=0.62AV2gh, or the actual value of E is about 0.62 of its theoretical amount.

Three Wire D. C. System

What is the Three Wire D. C. System, and why is it used?-F. I. R.

The Three Wire Direct Current System consists of two constant potential generators connected in series and each of 100 volts, as shown in the accompanying sketch. Line A is connected to the positive brushes of the generator 1, and line C is connected to the negative brushes of generator 2, while line B is connected to the junction of the negative brush of 1 and the positive brush of 2. It is called the neutral wire and gives a voltage of 110 volts between A and B and between B and C, while there is a difference of potential of 220 volts between A and C.

The lamps shown are 110 volt incandescent lights and are connected between the neutral and either of the outside wires. To obtain the best conditions of operation, the system should be balanced as nearly as possible. In case the system is unbalanced, the difference of potential between the two outside wires causes a current to flow in the neutral wire. In order to avoid upsetting the balance on

amount of power. This allows a great saving of copper for a given line loss in the transmission of a certain amount of power. Since the current is only onehalf as great, the line loss I2R will only be one-fourth as great for a given resistance. That is, the resistance of the line may be four times as great for the same line loss so that only one-fourth of the copper will be required for the outside wires, and if the neutral wire is the same size, only three-eighths of the amount of copper of a two-wire system operating under similar conditions will be necessary for this three-wire system.

Fast Working Bit

I understand that there is a modern bit brace on the market having a supplement that will permit boring holes twice as fast near a wall or corner as can be done with the ordinary brace. Can you give me any information about this?-W. L. H.

The diagram shows such a brace. The general construction of the device is the same as any first-class brace, less the supplement, which consists of a sleeve at casing b with its interior parts, which are shown in Fig. 3, where casing and parts are cut in center, also rod F, Fig. 2, which is used for connecting slotted end of short shaft C with slotted end of spindle G.

A shaft, No. 1, Fig. 3, is securely fastened in handle a, said shaft terminates in casing b, and carries a ratchet wheel No. 1, and bevel gear No. 3, which rotate freely and together on shaft, which also carries radial bevel gears No. 4-4. Forked shaft C carries bevel gear No. 5

and is securely fastened thereto. Casing b carries pawls d, one on each side, and swivels e for disengaging pawls d from ratchet wheel No. 2.

When a hole is to be bored in a place where the full sweep of the brace can not be used, the operator connects short shaft C with spindle at G.as shown, and engages pawl d on casing, and pawl opposite to d 1 on spindle, with their respective ratchets.

When the handle a is held stationary and the sweep is oscillated in the same manner as an ordinary brace, using only part of sweep, the result is, owing to the arrangement of parts, that the spindle H will turn continuously to the right (also left when desired) during both forward and backward motion of the sweep.

If the sweep is oscillated 1/25th of its entire circumference holes can be bored in narrow and contracted spaces.

An Unusual Photo THE photographer who succeeded in

getting a picture of Lancia as he ran the second heat of the five-mile open championship in the Daytona-Ormond races had good reason to be proud of his achievement. When the picture was taken the 110 horse power racer was moving at the rate of about 150 feet per second, and it would seem impossible to photograph an object moving at such a terrific speed. However, as the shutter was set to give an exposure of only about one one-hundredth of a second, the distance traversed by the machine while the picture was taken was only about a foot and a half. This was still further reduced by the distance of the machine from the camera and the position of the photographer almost in its path. The great difficulty in taking the picture was not with the mechanism of the camera, as the speed of the shutter was ample, but lay with the handling by the photographer. Had he missed by a quarter of a second the precise instant at which he released his shutter, he would have had only a streak of black across his picture or the machine would have been far behind him down the beach. Such precision is almost beyond human. control, and the remarkable picture here shown must be ascribed to great good luck.

The beach shown is

famous as the most magnificent race course in the world, either artificial or natural. It is over 150 yards wide at low tide, and presents an unbroken stretch of twenty miles, absolutely level, smooth and hard. The races are held when the tide is out, and between each series of events the waters of the Atlantic completely renew the track. A closer inspection of the tracks which may be seen upon the beach would show that the great racing machines almost leave the ground in their course and fly through the air, as for distances of eight or ten feet the tracks can hardly be seen upon the sand. To the right may be seen the peculiar track made by a machine which "skidded" in turning upon the beach. These races are very popular with tourists from all parts of the country, who come to enjoy the milder climate. The long stretch of the fine, dustless course gives excellent opportunity for observation.