The process of drawing a thread by this method is exactly similar to the operation already described in connection with the arrow method. Though short thick threads form the product generally obtained from the catapult, it must not be supposed that thin threads cannot be obtained in this way. If a short length of a very fine needle be heated, it will be found to yield threads quite fine enough for ordinary suspension purposes, but naturally not so uniform as those obtained from the 40-foot lengths obtainable by the bow-and-arrow method. It is easy to make spiral quartz springs resembling watch balance-springs by means of the catapult. All that is necessary is to see that the quartz is rather unequally heated before the shot is fired. In the future it is by no means impossible that such springs may have a real value, for the rigidity of quartz is known to increase as temperature rises. Hence it is probable that the springs would become stiffer as temperature rises, even though they work chiefly by bending, and little or not at all by twisting. As this is the kind of temperature variation required to compensate an uncompensated watch balance wheel, it may turn out to have some value. Sec. 87. Drawing Threads by the Flame alone. A stick of quartz is drawn down to a fine point, and the tip of this point is held in the blow-pipe flame in the position shown in Fig. 70. Fig. 70. The friction of the flame gases is found to be sufficient to carry forward the fused quartz and to draw it into threads in spite of the influence of the capillary forces. If a sheet of paper be suspended at a distance of two or three feet in front of the blow-pipe flame, it will be found to be covered with fine threads tangled together into a cobwebby mass. As this method is an exceedingly simple one of obtaining threads, I have endeavoured to reduce it to a systematic operation. A sheet of cardboard, about two feet square, is painted dead black and suspended horizontally, painted side downwards (Fig. 70, A), at a height of about two feet above the blow-pipe flame. The latter is adjusted so as to point almost vertically upwards and towards the centre of the cardboard. A few half-inch pins are thrust through the card from the upper surface and pushed home; about one dozen pins scattered over the surface will be sufficient. Their object is to prevent the threads being carried away round the edge of the screen. The flame from the jet described so often is fed from gas bags weighted to about eighty pounds per square foot of (one) surface, i.e. "4-foot" bags require from three to four hundredweight to give an advantageous pressure. [Footnote: The resulting threads were really too fine for convenient manipulation, so that unless extremely fine threads are required it will be better to reduce the pressure of the gases considerably.]