Proportion of Salt in 100 parts of water 0 Boiling-point 212 deg. " " 1/33 " 213.2 " " 2/33 " 214.4 " " 3/33 " 215.5 " " 4/33 " 216.6 " " 5/33 " 217.9 " " 6/33 " 219.0 " " 7/33 " 220.2 " " 8/33 " 221.4 " " 9/33 " 222.5 " " 10/33 " 223.7 " " 11/33 " 224.9 " " 12/33 " 226.0 When the salts in solution amount to 12/33, the water is saturated. It has also been ascertained that, when a solution of 4/33 is attained, incrustation of the substances commences on the boiler. Hence, it is a rule with engineers to expel some of the boiling water, when the thermometer indicates a temperature of 216 deg., and introduce some more cold water, in order to prevent incrustation, which not only injures the boiler, but opposes the passage of heat to the water. The thermometer used for this purpose should be very accurately graduated, and the scale must be considerably higher than, though it need not read much below 212 deg. _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 deg., 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. [Illustration: Fig. 74.] 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.