CONSTRUCTION.

The mercury tube should be made first.  Take a piece of glass tubing 20 inches long, and bend it at a point 9 inches from one end after heating in a spirit flame.  The legs should be kept as parallel as possible.  Lay the tube, while the heated part is still pliant, on a flat surface, the bend projecting over the edge, So that the two legs shall be in line.  When the glass has cooled, bend over two inches of the longer leg to an angle of about 45 degrees.

A standard for the tube is now made out of one-inch wood.  Hollow out a bed in which the tube shall lie and be completely protected.  To the right of the tube the standard is notched to take a small bottle.  The notch should be slightly narrower than the diameter of the bottle, and have its sides hollowed out to fit.

Halfway up the tube draw a zero mark across the standards, and above this a scale of inches in fractions on both sides.  Each inch represents 1 lb. pressure.

The cork of the bottle must be pierced with a red-hot wire for two glass tubes, one of which is bent over for the blowing tube.  Both tubes should be pointed at the bottle end so that they may enter the cork easily.  Make the top of the cork air tight with sealing-wax.  The purpose of the bottle is to catch any mercury that might be sucked out of the tube; one does not wish mercurial poisoning to result from the experiments.  Also it prevents any saliva entering the mercury tube.

When the latter has been secured to the standard by a couple of slips of tin nailed to the front, connect it up with the bottle, and fill it up to the zero mark with mercury poured in through a small paper funnel.

The open end of the tube should be provided with an inch of tubing.  Clips placed on this and on the rubber connection between tube and bottle will prevent the escape of mercury should the apparatus be upset when not in use.

The average blowing pressure of which the lungs are capable is about 1-1/2 lbs. per square inch; inspiration pressure without mouth suction about 1 lb. per square inch; suction pressure 2-1/2 to 3 lbs. per square inch.

Caution.—­Don’t ask people with weak lungs to try experiments with the apparatus described in this chapter.

XXXI.  HOME-MADE HARMONOGRAPHS.

Have you ever heard of the harmonograph?  If not, or if at the most you have very hazy ideas as to what it is, let me explain.  It is an instrument for recording on paper, or on some other suitable surface, the figures described by two or more pendulums acting in concert.

The simplest form of harmonograph is shown in Fig. 168.  Two pendulums are so suspended on points that their respective directions of movement are at right angles to one another—­that is, pendulum A can swing only north and south, as it were, and pendulum B only east and west.  On the top of B is a platform to carry a card, and on the upper end of A a lever is pivoted so as to be able to swing only vertically upwards and downwards.  At its end this lever carries a pen, which when at rest lies on the centre of the card platform.