2nd. Pressure Gauge.—The elasticity of gases augments by increase of temperature, and vice versa; it follows, therefore, that when steam is generated in a closed boiler, its temperature rises beyond the boiling temperature of 212°, owing to the increased pressure upon the water. The law connecting the pressure and the corresponding temperature of steam is the same as that upon which the boiling of fluids under diminished atmospheric pressure takes place. Hence, the indications of the thermometer become exponents of steam pressure. Engineers are furnished, in works on the steam-engine, with tables, from which the pressure corresponding to a given temperature, or the converse, can be obtained by mere inspection.
Fig. 74 represents the thermometer employed as a steam-pressure gauge. It is fitted in a brass case, with screw-plug and washers for closing the boiler when the thermometer is not in use. The scale shows the pressure corresponding to the temperature, from 15 to 120 lbs., above the atmospheric pressure, which is usually taken as 15 lbs. on the square inch.
CHAPTER XI.
INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
97. Hygrometric Substances.—The instruments devised for the purpose of ascertaining the humidity of the atmosphere are termed hygrometers. The earliest invented hygrometers were constructed of substances readily acted upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c., which all absorb moisture, and thereby increase in length, and when deprived of it by drying they contract. Toy-like hygrometers, upon the principle of absorption, are still common as ornaments for mantel-pieces. A useful little instrument of this class, formed from the beard of the wild oat, is made to resemble a watch in external appearance, and is designed to prove the dampness or dryness of beds: a moveable hand points out on the dial the hygrometric condition of the clothes upon which the instrument is laid.
Fig. 75.
98. Saussure’s Hygrometer, formerly used as a meteorologic instrument, but now regarded as an ornamental curiosity, is represented in fig. 75. Its action depends upon a prepared hair, fixed at one end to the frame of the instrument, and wound round a pulley at the other. The pulley carries a pointer which has a counterpoise sufficient to keep the hair stretched. By this means the shrinking and lengthening of the hair cause the pointer to traverse a graduated arc indicating the relative humidity.