Page:Popular Science Monthly Volume 39.djvu/646

628 construction of the ordinary instruments sold in the shops for domestic usage. Their low price would preclude that. They are made in large quantities, and their calibration is only approximate. They have been known to be as much as six or eight degrees out of the way; but that is much worse than the average. After one has watched the construction of a scientific thermometer, he wonders not that they should cost a couple of dollars or more, but rather that they can be sold at such a price.

For many purposes, such as the systematic observation of the weather, it is desirable to have thermometers which shall register the highest and lowest temperature reached. These maximum and minimum instruments require additional care on the part of the thermometer-maker. In the latest pattern manufactured for the Signal Service a decided improvement has been made in the self-registering device for maximum temperatures. The bore of the tube is greatly contracted at a point somewhat below the lowest reading that will probably be required, and the thermometer is usually placed in a horizontal position. Under the action of an increasing temperature, the mercury expands and forces itself through the very narrow opening. But when the temperature falls, the mercury will not pass through this opening, and all the shrinkage of the fluid in the bulb takes place below the contraction. As a consequence, the column of mercury remains stationary, and so records the highest temperature reached. By vigorous shaking the instrument is readily reset.

In the thermometer for registering the lowest temperature colored alcohol replaces mercury. A little rider of glass is so trimmed with fine hairs at each end that, while it does not fit in the tube with sufficient snugness to prevent its being pushed down the tube by the retreating meniscus at the surface of the alcohol, it will become wedged in place when the column ascends.

The special feature to be noticed in the manufacture of the thermometer is the individuality of the process. Each instrument is the subject of a separate operation. The same principle is applied in the manufacture of barometers and hydrometers. In the fabrication of the first, a glass tube is simply closed at one end and then filled with pure mercury, from which all the air has been expelled by boiling. Its subsequent marking and adjustment in a suitable frame are only matters of careful handling.

In the fabrication of hydrometers more special work comes in. The transparency of the material is not here an essential feature, although it is utilized and the graduation placed inside of the tube. The quality which renders glass particularly available for this service is its indifference to chemical reagents and its constant weight. The principle upon which hydrometers are constructed is familiar to all. In order that an object may float,