20 



THE VOYAGE OF H.M.S. CHALLENGER. 



IV. Amagat's Manometre a Pistons libres. 



The annexed sketch of the instrument (in which the large divisions shown on the 

 manometric scale correspond to decimetres), with the section given below, will enable 



the reader to understand its size and construction without any detailed description 

 beyond what is given in the instructions for setting it up. [The window FF, whose 

 position is nearly immaterial, occupies different positions in the. sketch and in the 

 section.] 



As already stated, the principle on which this instrument works is the same as 

 that of the Manometre Desgoffes, a sort of inverse of that of the well-known Bramah 

 Press. In the British instrument pistons of very different sectional area are subjected 



