144 VOYAGE TO MADAGASCAR* 



This pifton moves in a collar lined witli 

 tow, which is fqueezed clofe to it by means 

 of ikrews, and is furroundcd by a ring to 

 confine the fteam, and prevent the external 

 air from penetrating into the body of the 

 cylinder. 



When the fteam which rifes from the 

 boiler fills the body of the cylinder^ and 

 prefles equally on the upper and lower part 

 of the pifton, the expanfive force of the 

 vapour being then in equilibrio^ the piftoo 

 remains fixed at its degree of elevation ; but 

 the elaftic fluid fucceffivcly lofes its fpriog, 

 either in the upper or lower part of the 

 pifton^ in a very lliort time. By this ex- 

 cellent conftrudion, the rifing and adion 

 of the vapour are continual ; for after the 

 fleam is intercepted under the lo%ver part 

 of the piftoii, and the cold water injeded, 

 has occafioned a vacuum under it, the ex- 

 panfive force of the fteam pre (ling upon the 



pifloii 



