FAYERNBTS HTDROSTAT. 275 



below, as just stated, and communicates by a large 

 chimney, or shaft, with the upper compartment, or 

 hetween-decks. Between these is a tliird compartment, 

 or orlop-deck, which only communicates with the 

 others by means of stopcocks. All round the hold 

 and the orlop-deck runs a gallery, hermetically 

 closed, and connected with the former compartments 

 by stopcocks only. The lower part of the gallery 

 receives the ballast of the machine, whilst the upper 

 is filled with air or water as occasion requires. (Fig, 

 54.) 



Whilst the hydrostat floats, the hold and one por- 

 tion of the shaft are full of water ; the orlop-deck, its 

 gallerij, and the hetween-decks being full of air. A 

 lift and force-pnmp are found in the latter, where 

 the workmen would now be stationed. 



When it is wished to sink the hydrostat, the hatch 

 of the between-decks and tlie dooi* of the shaft are 

 closed hermetically. The pump is worked in such a 

 manner as to draw water from the exterior, and fill 

 the orlop-deck and its galleiy. A pipe furnished 

 with a stopcock allows communication between the 

 upper part of the orlop-deck and the hold. At the 

 same time that the latter is filling with compressed 

 air, the apparatus fills with water, gets heavier, and 

 ultimately sinks. The water which was in the hold 

 has, it is true, been expelled ; but the contents of 



