8EA-WATER AT VARIOUS DEPTHS. 45 



it to the surface with all that it contains in hermeti- 

 cally-sealed vessels. It is not possible to employ 

 empty vessels, such as are used for investigations 

 into the character of the atmosphere, contrived to 

 open at the required depths. The water would 

 either break the vessels or filter through them. On 

 their approaching the surface, any giises that had 

 been subject to the enormous powers of compression 

 of the superincumbent water, would dilate to an ex- 

 tent \\hich no vessel hermetically sealed could be 

 expected to resist. Biot has, to some extent, over- 

 come these difficulties in the following manner. 



He takes for his vessel a hollow cylinder of glass, 

 closed at one end by a solid plate of metal, so as to 

 resemble a pail. Like a pail also it has a handle, 

 with a cord attached, by which it is let down into 

 the sea. This pail being open to the surrounding 

 water, descends to any required depth without injury 

 from the pressure of the water ; and whenever the 

 operator pleases he pulls a second cord, attached 

 to the bottom of the pail by means of an inverted 

 handle, and this causes the vessel to tip over. By 

 the same cord he draws it to the surface. It must 

 now be observed that the metal bottom of the pail is 

 double ; one of its parts being fixed, the other move- 

 able like a piston, and capable of descending in 

 the cylinder by its own weight when the vessel is 



