The Boy Mechanic Part 71

The frame for the hour axis is about 12 in. long with a bearing at each end. The shaft which it carries is 1/4-in. steel, carrying the hour circle at one end, and at the other the frame for the declination axis which is similar to the other, but somewhat lighter. The declination axis is also of 1/4-in. steel, carrying at one end the declination circle and the pointer at the other.

The entire frame of the instrument is made of cherry and it will save the builder much time if he will purchase cherry "furniture"

which is used by printers and can be obtained from any printers'

supply company. It is best quality wood free from imperfections in straight strips one yard long and of a uniform width of about 5/8 in. As to thickness, any multiple of 12-point (about 1/8 in.) may be obtained, thus saving much work in fitting up joints. Fifty cents will buy enough wood for an entire instrument. All corners are carefully mortised and braced with small bra.s.s angle-pieces.

The frame is held together by small bra.s.s machine screws. After much experimentation with bearings, it was found best to make them in halves as metal bearings are usually made. The loose half is held in place by guides on all four sides and is tightened by two screws with milled nuts. A great deal of trouble was experienced in boring out the bearings until the following method was devised.

One hole was bored as well as possible. The bearing was then loosened and a bit run through it to bore the other. Finally, a piece of shafting was roughened by rolling it on a file placed in both bearings and turned with a brace. The bearings were gradually tightened until perfectly ground.

The declination axis must be perpendicular to both the hour axis and the line of sight over the pointer. To insure this, a positive adjustment was provided. The end of the shaft is clamped in a short block of wood by means of a bearing like the ones described.

One end of the block is hinged to the axis frame, while the other end is attached by two screws, one drawing them together, the other holding them apart. The axis is adjusted by turning these screws. Each shaft, save the one in the pipe, is provided with this adjustment.

The pointer is of two very thin strips placed at right angles and tapered slightly at each end. The clamp is attached as shown in the ill.u.s.tration. The eye piece is a black iron washer supported on a small strip of wood. The aperture should be 1/4 in., since the pupil of the eye dilates very much in darkness. The error due to large aperture is reduced by using a very long pointer which also makes it possible to focus the eye upon the front sight and the star simultaneously. The forward sight is a bright bra.s.s peg illuminated by a tiny electric lamp with a reflector to shield the eye. The pointer arranged in this way is a great improvement over the hollow tube sometimes used, since it allows an un.o.bstructed view of the heavens while indicating the exact point in question.

The circles of the instrument are of aluminum, attached to the shafts by means of wooden clamps. They were nicely graduated by a home-made dividing engine of very simple construction, and the figures were engraved with a pantograph. The reading is indicated by a cut on a small aluminum plate attached to a pointer. The hour circle is divided into 24 parts and subdivided to every four minutes. The figures are arranged so that when the instrument is set up, the number of hours increases while the pointer travels oppositely to the stars. The declination circle is graduated from zero to 90 deg. in each direction from two points 180 deg. apart.

It is, adjusted to read zero when the pointer and two axes are mutually perpendicular as shown in the picture.

To adjust the instrument it is set up on the iron pipe and the pointer directed to some distant object. All set screws, excepting those on the declination axis, are tightened. Then the pointer is carefully turned through 180 deg. and if it is not again directed to the same point, it is not perpendicular to the declination axis. When properly set it will describe a great circle. With the declination axis in an approximately horizontal position the place where the pointer cuts the horizon is noted. The declination axis is then turned through 180 deg., when the pointer should again cut at the same place. Proper adjustment will cause it to do so. It is desirable that the hour circle should read approximately zero when the declination axis is horizontal, but this is not necessary for a reason soon to be explained. All these adjustments, once carefully made, need not be changed.

In using the instrument the hour axis can be directed to the north pole by the following method. Point it approximately to the north star. The pole is 1 deg. and 15 min. from the star on a straight line from the star to "Mizar," the star at the bend of the handle in the Big Dipper. Turn the hour circle into a position where the pointer can describe a circle through "Mizar." Only a rough setting is necessary. Now turn the pointer so that a reading of 88 deg. 45 min. shows on the declination circle on that side of 90 which is toward "Mizar." When this is done, clamp both axes and turn the shafts in the base until the pointer is directed accurately to the north star. It is evident from a study of the picture that the position of the small pointer which indicates the reading on the hour circle is not independent of the way in which the tripod or pipe is set up. It would then be useless to adjust it carefully to zero when the pointer cuts the "zenith" as is done with a large equatorial. Instead, the adjustment is made by setting the clock or watch which is part of the outfit. The pointer is directed to Alpha, Ca.s.siopiae, and the hour reading subtracted from 24 hours (the approximate right ascension of the star) gives the time which the clock should be set to indicate.

All of these settings should require not more than five minutes.

To find a star in the heavens, look up its declination and right ascension in an atlas. Set the declination circle to its reading.

Subtract the clock time from the right ascension (plus 24 if necessary) and set the hour circle to the result. The star will then be seen on the tip of the pointer.

To locate a known star on the map, turn the pointer to the star.

Declination is read directly. Add the clock time to the hour reading to get right ascension. If the result is more than 24 hours, subtract 24.

** A Ground Gla.s.s Subst.i.tute [255]

Ordinary plain gla.s.s coated with the following mixture will make a good ground gla.s.s subst.i.tute: Dissolve 18 gr. of gum sandarac and 4 gr. of gum mastic in 3-1/2 dr.. of ether, then add 1 2-3 dr.

benzole. If this will be too transparent, add a little more benzole, taking care not to add too much. Cover one side of a clear gla.s.s and after drying it will produce a perfect surface for use as a ground gla.s.s in cameras.

--Contributed by Ray E. Strosnider, Plain City, Ohio.

** A Miniature War Dance [255]

A piece of paper, 3 or 4 in. long, is folded several times, as shown in the sketch, and the first fold marked out to represent one-half of an Indian. Cut out all the folds at one time on the dotted line and you will have as many men joined together as there were folds in the paper. Join the hands of the two end men with a little paste so as to form a circle of Indians holding hands.

The next thing to do is to punch holes in heavy cardboard that is large enough to cover a pot or stew pan, and

[Ill.u.s.tration: Indian War Dance]

partially fill the vessel with water. Set this covered vessel over a heat and bring the water to a boiling point and then set the miniature Indians on the perforated cover. The dance will begin.

If the Indians are decked out with small feathers to represent the head gear and trailing plumes, a great effect will be produced.

--Contributed by Maurice Baudier, New Orleans, La.

** Saving an Engine [255]

Turning the water on before starting the gas engine may prevent breaking a cylinder on a cold day.

** OLD-TIME MAGIC [256]

Removing 36 Cannon b.a.l.l.s from a Handbag

The magician produces a small handbag and informs the audience that he has it filled with 20-lb. cannon b.a.l.l.s. He opens up the bag and takes out a ball which he pa.s.ses to the audience

[Ill.u.s.tration: b.a.l.l.s Made of Spring Wire]

for examination. The ball is found to be the genuine article. He makes a few pa.s.ses with the wand and produces another ball, and so on until 36 of them lie on the floor.

In reality the first ball, which is the one examined, is the real cannon ball, the others are spiral-spherical springs covered with black cloth (Fig. 1). These b.a.l.l.s can be pressed together in flat disks and put in the bag, Fig. 2. without taking up any great amount of s.p.a.ce. When the spring is released it will fill out the black cloth to represent a cannon ball that cannot be distinguished from the real article.

--Contributed by J. F. Campbell, Somerville, Ma.s.s.

** A Rising Card Trick [256]

A rising card trick can be accomplished with very little skill by using the simple device ill.u.s.trated. The only

[Ill.u.s.tration: Card Slips from the Pack]

things needed are four ordinary playing cards and a short rubber band. Pa.s.s one end of the rubber band through one card and the other end through the other card, as shown in the ill.u.s.tration, drawing the cards close together and fastening the ends by putting a pin through them. The remaining two cards are pasted to the first two so as to conceal the pins and ends of the rubber band.

Put the cards with the rubber band in a pack of cards; take any other card from the pack and show it to the audience in such a way that you do not see and know the card shown. Return the card to the pack, but be sure and place it between the cards tied together with the rubber band. Grasp the pack between your thumb and finger tightly at first, and by gradually loosening your hold the card previously shown to the audience will slowly rise out of the pack.

--Contributed by Tomi O'Kawara, San Francisco, Cal.

** Sliding Box Cover Fastener [256]

[Ill.u.s.tration: Box with Fastener]

While traveling through the country as a watchmaker I found it quite convenient to keep my small drills, taps, small brooches, etc., In boxes having a sliding cover. To keep the contents from spilling or getting mixed in my case I used a small fastener as shown in the accompanying ill.u.s.tration, The fastener is made of steel or bra.s.s and fastened by means of small screws or tacks on the outside of the box. A hole is drilled on the upper part to receive the pin that is driven into the sliding cover. This pin should not stick out beyond the thickness of the spring, which is bent up at the point so the pin will freely pa.s.s under it. The pin can be driven through the cover to prevent it from being pulled entirely out of the box.

--Contributed by Herm Grabemann, Milwaukee, Wis.

** How to Chain a Dog [257]

A good way to chain a dog and give him plenty of ground for exercise is to stretch a clothesline or a galvanized