CORKS. CORK-BORERS. 



9 



but this happens because air, as will be shown further 

 on, is a very compressible body, and its volume is 

 slightly diminished in these experiments by the pressure 

 exerted by the water in the jar, and in the funnel re- 

 spectively. 



Corks suitable for such experiments must be selected with care 

 from a corkcutter's stock. They should be close grained, free from 

 holes, and rather soft. A cork intended for a particular orifice 

 should always be of somewhat greater width than the hole itself ; 

 by gentle hammering, or by rolling it between a small board and 



iji BRAKY 



UNUV KUMTY <>\' 



FIG.. 10(| red size). 



the table, it may be softened so as to fit tightly into the neck of a 

 glass vessel without breaking it. A cork of suitable size, if not 

 readily found, may be cut from a larger one with a very sharp 

 knife, which is moved like a saw, or it may first be cut roughly, 

 and then shaped by filing with a flat file. Holes in corks are 

 best made by means of proper cork-borers (fig. 10). These are 

 tubes of thin brass, open at both ends, provided with a handle at 

 one extremity, and sharpened at the other ; a set usually contains 

 6 or 10, varying in width from 3 to 15 mm . In penetrating through 

 the cork the tube is turned with a slight pressure, and the plug 



