20 



THE VOYAGE OF H.M.S. CHALLENGER. 



External Pressure Gauge. 



But it was necessary not merely to measure accurately the pressure applied, but 

 also, for the sake of the thermometers, to provide that the pressure should not be carried 

 too far ; and for that purpose it was indispensable to have an exterior indicator of pressure. 



This was furnished by a thin cylindrical steel tube enclosed in a cavity bored in a 

 large block of iron, the interior of the steel tube being full of mercury and the narrow 

 space between it and the large iron block also full of mercury. This exterior space was 



connected with the pressure apparatus. The pressure then throughout the whole of the 

 space exterior to the steel cylinder was the pressure in the pump. The steel cylinder 

 was therefore compressed from the outside. In the neck of the steel cylinder, which 

 was screwed into the surface of the block, there was luted a vertical glass tube. It 

 was exposed to no pressure, but the mercury in it rose, by the compression of the 



