OBJECTS.] INTRODUCTORY. 2 3 



material substance, resists the intrusion of other mat- 

 ter into the place which it occupies. But many things, 

 though they resist, can be easily squeezed or com- 

 pressed into a smaller volume. This, however, is not 

 the case with water, which like other liquids, is almost 

 incompressible : that is to say, an immense pres- 

 sure is needful to cause its volume to diminish to any 

 appreciable extent. It may seem strange that anything 

 so apparently yielding as water should yet be almost 

 as difficult to squeeze as so much iron ; but the apparent 

 yieldingness of water is due to the ease with which 

 it changes its shape ; and, if water is prevented from 

 changing its shape, it is very difficult to drive its parts 

 closer together. It has been ascertained that if water 

 is confined in a closed space, a pressure amounting to 

 fifteen pounds on the square inch diminishes its volume 

 by only ^ Tr ^ TJ th part. Take a common syringe, and 

 having seen that the plug or piston fits the cylinder 

 of the syringe well, put the nozzle into water and draw 

 the piston up. Then turn the nozzle upwards and 

 push upon the piston till a little of the water squirts 

 out, so as to make sure that the cylinder contains 

 nothing but water. Now put your finger on the 

 opening of the nozzle firmly, so as to stop any water 

 from passing out, and then try to push the piston down. 

 You will find that you cannot make it stir without great 

 force ; and, if the piston moves appreciably, it will be 

 because some of the water has escaped by the sides 

 of the piston. In fact, if the piston presented a square 

 inch of surface, and fitted accurately, and the column 

 of water in the cylinder were one inch long, it must 

 be pressed down by a weight of 30,000 pounds (about 

 thirteen tons) to make it move one-tenth of an inch. 

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