PRINCIPLE OF THE BAROMETER. 



445 



The mechanism of the hand and dial is shown in the diagram 

 which occupies the centre of the illustration. For convenience, 

 sake the mercury column is mostly employed, but several 

 Water Barometers, some thirty feet in length, have been con- 

 structed. 



On the left hand is seen a boy engaged in sucking an egg. 

 The plan employed is simple enough. A tolerably large hole 

 is made at one end, and a very small one at the other. The 

 yolk having been broken up by a long needle, or similar imple- 

 ment, the larger hole is placed to the lips, and, suction being 

 used, the contents pass into the mouth. 



Were it not for the hole at the end opposite the mouth, it 

 would be impossible to extract the contents, but the air rushes 

 through the aperture, and so forces out the contents of the egg. 



Above is a representation of the way in which Sugar-cane is 

 sucked. The reader probably knows that the Sugar-cane, like 

 the wheat- stem, has knots at certain intervals, which divide 

 the cane into a number of separate parts. 



There is quite an art in sucking the Sugar-cane. If a joint 

 be cut off, and the lips applied to the end, not a drop of the 

 sweet juice would be extracted. But if a notch be cut close to 

 the joint, as shown in the illustration, the air can gain access, 

 and then the juice flows easily enough. 



It has already been mentioned that air expands when heated. 

 The same rule holds good when applied to other objects, such 



BOILING WATEE. 



THEBMOMETER. 



as the various liquids, metals, &c. A very familiar example of 

 this fact is the " boiling over " of water, when the vessel has 



