Voyage to Madagascar. 145 



pifton caufes it to defcend ; and becaufe the 

 communication of the fteam with the upper 

 part is intercepted^ and that with the lower 

 re-eflabliflied, the pifton cannot fait to rife 

 with the fame force, if a vacuum be made 

 in the upper part of the pifton by the injec- 

 tion of cold water. Such are the late im- 

 provements made in fteam engines by MeQrs, 

 Watt and Bolton. Thofe which we have 

 in France, according to the lateft improve- 

 ments of thefe able mechanics, caufe a va- 

 cuum in the lower part of the pifton, and 

 not in the upper. Meffrs. Watt and Bol- 

 ton's principal objed in this conftrudion 

 was, to prevent the body of the engine, 

 which receives the fteam, from cooling. 

 For this reafon, they preferred the a£tion of 

 fteam to that of the preffure of the exter- 

 nal airi and, in order that the water injeded 

 may not cool the cylinder, when it caufes 

 L a vacuum. 



