ATMOSPHERIC PRESSURE. 329 



make the facing of much harder material than the stone, we 

 should make an advance in the miller's art that would render 

 the millstones of the future as far superior to those of the 

 present as are our present millstones to the hand " quern " of 

 the Kafir women. 



Yet another improvement has to be made. Would it be 

 possible to construct a millstone which should not only retain 

 its facing, but possess the power of renewing itself in propor- 

 tion as it is worn out? This property is found in the Elephant's 

 tooth, and the illustration will give a tolerably good idea of 

 the simple and beautiful mechanism by which it is brought 

 into operation. 



The tooth, instead of being one solid mass, consists, as I have 

 already stated, of a series of plates set side by side. These 

 plates are so constructed that they are more worn away in 

 front than behind. In proportion as they are worn, a new tooth 

 is built up behind the old one, and gradually pushes off the old 

 one. Now, if we could only construct millstones with such 

 properties, we should possess an absolutely perfect instrument. 



Pressure of Atmosphere. 



There are many useful inventions which depend on the 

 weight of the atmosphere and the creation of a more or less 

 perfect vacuum. There is, for example, the common Pump, 

 which raises water simply by the action of the atmosphere. A 

 pipe passes into the water, and in that pipe an air-tight piston 

 is inserted. When the piston is drawn upwards a vacuum is 

 formed, and the water is at once forced into it by the pressure 

 of the atmosphere. 



Then there is the graceful and useful Napier Coffee-making 

 Machine, consisting of a glass globe, and vase of the same 

 material. 



Coffee and boiling water are put into the vase, and some hot 

 water into the globe. The two are then connected with the 

 tube, and under the globe is placed a spirit-lamp. Presently 

 the water in the globe boils, expelling the air and filling the 

 globe with steam. The lamp is then removed, and the steam 

 in the globe is condensed, leaving a vacuum. The pressure of 

 the atmosphere then comes to bear upon the coffee in the vase, 



