A^ignst 19, 1880] 



NATURE 



^^7 



used for a drawing-room aquarium will answer capitally 

 for this purpose ; but if no deep glass vessel is available, 

 a pan or tub of stone-ware or of tin-ware will serve the 

 purpose. The disk of card should be pressed against 

 the lower end of the lamp-chimney (as in Fig. 8) by 

 pulling up the thread through the glass tube. If it is 

 then lowered into the water in the glass trough, the 

 upward force of the water outside pressing up against 

 the card disk will keep it against the end of the lamp- 

 glass. The deeper it is plunged the more tightly is it 

 pressed up against the end of the tube, for the pressure 

 of the liquid becomes greater and greater as the depth 

 of the disk below the surface is increased. A case of 

 downward pressure is even more simply shown. Take 

 the lamp-chimney in your hand and hold it vertical as 

 before, and fix to the lower end another disk of card, this 

 time fixing it to the bottom of the glass by means of soft 

 bees'-wax or of a little stiff tallow. Now pour in some 

 water from above. At first the disk is held on by the 

 wax, and you may pour in water until the chimney is 

 perhaps half full. But as you go on pouring in water the 



on opening a tap in a lower storey the water rushes out 

 with very great force, so grea% perhaps, that we cannot 



depth of the water inside gets greater and greater, and 

 the pressure exerted by the column of liquid becomes 

 also greater, until the adhesive force of the wax is over- 

 come, and the water bursts off the card and rushes out. 

 This second experiment may be combined with the first 

 one, as is shown in Fig. 8. After having lowered the 

 empty lamp-chimney closed by the card disk into the 

 rough of water, slowly pour in water into the inside. 

 As long as the level of the water outside is higher than 

 that of the water inside, the outer pressure upwards will 

 be greater than the inner pressure downwards ; but as 

 soon as enough water has been poured in to raise the 

 inner level to that outside, the internal and external 

 pressures will be equal, and when a few more drops are 

 added inside the card will be forced away. The fact that 

 liquid pressure depends upon the height of the column 

 of liquid that is pressing, is made familiar to us in the 

 arrangements for supplying our houses with water ; for 

 when the cistern is at the top of the house we find that 



possibly stop it with our hand, however tightly we press 

 it against the mouth of the tap. 



Another important law of liquid pressure, not so easy 

 of illustration without apparatus, is the famous principle 



