EXPERIMENTAL STUDY OF HEAT LEAKAGE. 749 



pressure. Heat is supplied to the boiler by a gas burner beneath it 

 {GB, Fig. 6). Special water level indicators are necessary at the 

 higher pressures, gauge glasses being impracticable. Two different 

 types have been used, one being that regularly supplied by the makers 

 of the boiler. This device, which is very reliable, indicates whether 

 the water level is below or above a not very sharply defined position, 

 but requires a rather large variation in level for its operation. It 

 was found desirable to have a sharper regulation of water-level than 

 this appliance affords, because of the danger of introducing moisture 

 into the steam if the level is raised too high. To secure this, a float 

 enclosed in a vertical steel tube has been used. The float is of glass, 

 copper-plated and then silver-plated. It completes an electrical 

 circuit through a relay and thereby turns off an otherwise lighted 

 incandescent lamp, when the surface of the water is raised to a certain 

 definite level. The silver-plating is necessary to secure reliability 

 of contact; it was found necessary also to silver-plate the inside of 

 the containing tube. The only difficulty with this appliance arose 

 from the gradual penetration of the w^ater to the interior of the float 

 through small flaws in the metal-plating — a rather striking example 

 of the slow solvent action of hot water upon glass. 



Pressure regulation is obtained by means of the hack-gauge G2, 

 Fig. 6. The pointer of this gauge makes an electrical contact between 

 platinum terminals when the boiler pressure rises to the desired value. 

 The completion of this circuit opens, through a relay, an electro- 

 magnetically operated valve which admits air under pressure to one 

 side of a U-tube containing mercury. The ensuing rise of the mercury 

 level in the other side of the U-tube shuts off a portion (or, if desired, 

 all) of the supply of gas to the burner. The electro-magnetic valve 

 closes again when the pressure has fallen sufficiently to permit the 

 platinum contacts mentioned to separate. 



As is indicated in Figs. 5 and 6, the steam passes from the boiler to 

 the primary superheater (Si in Fig. 5; Sin Fig. 6). This superheater 

 consists of six 30-inch lengths of two-inch double strength wrought 

 iron pipe, joined in series by cast-steel return bends, the whole capped 

 at each end with cast-steel caps, tapped to fit the smaller entrance 

 and exit pipes. The pipe connecting the boiler and superheater is 

 heavily lagged, and is heated, throughout its length, by the hot gases 

 from either the boiler furnace or the superheater furnace SF (Fig. 6). 

 The superheater and its furnace are enclosed in an asbestos box, 

 which is shown in part in Fig. 6, but not in Fig. 5. The total 

 volume of the primary superheater is about 0.33 ft. 



