628 THE POPULAR SCIENCE MONTHLY. 



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 approxi- 

 mate. They have been known to be as much as six or eight de- 

 grees out of the way ; but that is much worse than the average. 

 After one has watched the construction of a scientific thermome- 

 ter, 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 low- 

 est 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 contrac- 

 tion. As a consequence, the column of mercury remains station- 

 ary, 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 instru- 

 ment 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 adjust- 

 ment 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 avail- 

 able 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, 



