402 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



IV. SAFETY APPLIANCES. 



We have supposed the boiler properly filled with water, which, 

 when heated to the necessary temperature, furnishes to the steam-space 

 a certain quantity of steam at the required pressure. 



It is of the utmost importance that the level of the water should 

 not sink too low in the boiler, and that it should not rise in it above a 

 certain limit : in either case a risk is run, which is one of the most 

 frequent causes of the explosion of boilers. To obviate this, or at 

 least to indicate at any moment the exact level of the water in the 

 boiler, an appliance is used called the water-gauge. 



Thus you may always see on the outside of a boiler fully exposed 

 to view a glass tube, I, which communicates by its two ends with the 

 interior of the boiler (Fig. 276). The water has access to this tube, 

 and stands there in virtue of the law of equilibrium of liquids. 



A temporary excess of heat, or the bad working of the feed-pipe 

 owing to a sudden accident, might quickly lower the level and sur- 

 prise the engineer while he is occupied elsewhere. The water-gauge 

 would then be of no avail. It is necessary to add one or other of 

 the various systems of floats, which indicate the insufficient height of 

 the level by making a noise. Such are, for example, the alarm float 

 and the magnetic float. 



A float (it is generally a hollow metal ball) rises and falls with 

 the level of the water in the boiler. It is supported by a rod, which 

 forms one arm of a lever turning about a fixed point ; the other arm 

 supports a counterpoise. Within the proper limits of the water level 

 the rod holds a valve against the opening into a pipe in communica- 

 tion with the outer air. If the level of the water falls below these 

 limits, the float falls with it, and causes the valve to open. The 

 steam escapes by the tube and emerges by an annular orifice, where 

 it encounters the sharp edge of a bell, A, which it causes to vibrate so 

 as to produce a very intense and prolonged sound. 



The stoker is warned of the danger by the unusual sound ; and 

 hence the name alarm-float given to this apparatus. 



The dial gauge (F, Fig. 276) is formed of a disk, which a chain, 

 passing round the grove of a pulley attached to the dial, sustains and 



