CHAP. IL] AREOMETERS OR HYDROMETERS. 37 



by winding round interior and exterior bobbins ; the movement is 

 therefore continuous, whilst in the ordinary press the compression is 

 effected by successive strokes. But by the side of those advantages 

 there are inconveniences, which M. Tresca thus sums up in an other- 

 wise favourable report to the "Societe d'encouragement pour 1'industrie 

 nationale" : 



" To make room for the interior bobbin, a much larger capacity 

 must be given to the body of the press ; to transmit the movement 

 an aperture must be made for the arbor, and this aperture must be 

 furnished with a very thick tow casing ; the same remark also applies 

 to the aperture by means of which the wire is introduced, which 

 must not allow any liquid to ooze out, as otherwise the press might 

 be emptied and great diminutions of pressure take place during the 

 working." 



According to M. Tresca, the use of this new press would be especially 

 advantageous in smaller mechanical operations. In great undertakings 

 however serious difficulties would be met with in its use. 



II. AREOMETERS OR HYDROMETERS. 



The story of Archimedes coming out of his bath and running 

 through the streets of Syracuse, crying out, Evpyfca, evprjKa, " 1 have 

 found it ! I have found it ! " is well known. He referred to a pro- 

 blem which King Hieron had desired him to solve. It was necessary 

 to determine whether in a crown delivered to this tyrant by a gold- 

 smith, as pure gold, any other metal had been introduced. The dis- 

 covery of the principle of hydrostatics, which is named after the im- 

 mortal geometer, put him in the way of accomplishing this, and he 

 discovered that a certain quantity of silver had been mixed with the 

 gold in the making of the royal diadem. It is said that Archimedes 

 made little use of the practical applications of geometry and the 

 sciences ; but he was far from neglecting them : numerous inventions 

 of this kind are on record due to his genius. To him is attributed 

 the invention of areometers or hydrometers, instruments based 

 directly on the principle that all bodies immersed or floating in a 

 liquid displace, when equilibrium is established, a volume of liquid 



