356 EXPLANATION OF THE PLATES. 



o, Floats, with indexes, for shewing the height of the water in the boilers. 



p, Glass water-gauges, through which the exact height of the water is seen. 



q, Gauge cocks, below which the water is never supposed to descend. 



r, Steam chests. 



*, Safety valve box, containing the two safety valves, with the waste steam pipe 



rising from the same. 

 s, Waste steam pipe. 

 t, Pipes for conveying the water overboard from the safety valve box, and likewise 



that resulting from the condensation of the steam in the globe, or other con- 

 trivance at the head of the waste steam pipe. 

 u, Safety valve levers : accessible*at the front of the boilers, the safety valves being 



lifted by other levers fitted to the same axes, within the steam chest. 

 v, Man holes, with their covers in place. 

 w, Communication valves, having screwed spindles, which are acted on from above 



by two levers, for the purpose of shutting off either boiler from the main steam 



pipe. 



a, Main steam pipe. 

 y, Reverse valves, to prevent the boilers collapsing by the pressure of the atmosphere, 



which, in case of a vacuum being formed, presses the valves inwards to restore 



the equilibrium. 

 z, Vacuum pumps, the rods of which are attached to levers on deck, by working 



which, on charging the boilers with water from the sea, an exhaustion is effected 



which causes the water to rise within, from the pressure of the atmosphere 



without, and thus saves the labour of forcing many tons of water into them. 



The time occupied in fully charging the boilers of the " Medea " by these pumps 



was usually from fifteen to twenty minutes. 

 z, Blow-off pipes, with a nozle to each boiler. 



The same description applies generally to Plate XXII. of the African's boilers, 

 which are fitted with precisely similar appendages, with the exception of the 

 vacuum pumps. 



