Page:Popular Science Monthly Volume 39.djvu/642

624 mass, and a second workman attaches the end of his iron rod or "punty" to the free end of the ball. The blower remains stationary, while the second man walks away from him, carrying his punty with him. In this way the mass of glass is drawn into a long tube, perhaps fifty feet long, the bubble of air preserving a fine opening throughout the entire length of the tube. In the better thermometers, the tube is somewhat flattened, so as to make the thread of mercury more visible, and a background of opaque white glass is added for the same purpose. These modifications are made more easily, perhaps, than one would imagine. By flattening the ball of glass before it is drawn into a tube, the elliptical cross-section is secured, while a string of opaque glass welded on to the still plastic ball becomes elongated into a thin plating on one side of the tube.

It is impossible in this way to secure tubes of absolutely uniform bore, but the inequalities are much less than one would suppose. For ordinary instruments the variation may be neglected. The tubes are then cut into convenient length and sent to the workshop of the thermometer-maker. One can readily pass a whole morning in the little room where he works, for there is a certain interest attaching to so individual a task as this which is not found in more wholesale production. The instrument-maker sits on a high stool before his work-table, his principal tool being a conveniently arranged blowpipe. This is not the hand and mouth tool used by mineralogists and jewelers, but is a permanent blowpipe fed by gas and operated by a blast of air.

The first operation is that of forming the bulb. In the better instruments this is made out of a separate piece of glass and is then attached to the tube. In this case the bulb is made cylindrical in form, so as to afford large capacity without too great diameter. In the less expensive thermometers, the bulb is formed directly on the end of the tube itself. The glass is first fused in the blowpipe flame until the end is entirely sealed. A short rubber hose with a small rubber ball on the opposite end is then slipped over the open end of the thermometer-tube. The sealed end of the tube is again softened before the blowpipe, and then, by simply pressing the rubber ball, the air forces the plastic glass into a symmetrical bulb. It is a pretty little operation, for the glass responds so delicately to the thought of the workman.

It is found that glass undergoes a slow contraction during a period of two or three years, and, where great accuracy is desired, the tube must be put away for that time to season.

The bulb and tube are now to be filled with mercury. The tube is much too fine to allow the mercury simply to be poured into the bulb. Indirect means must be used. The open end of the tube is softened and quickly blown into a large bulb, while