12 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



FIG. 6. A Water-level. 



of them, we shall find that as soon as the water has come to 

 rest it will stand at the same level in all the tubes, however 

 different the form of the vessels may be. It is this property 

 of liquids which is utilised in the construction of the water- 

 level. Its construction and use will be easily understood by a 

 glance at Fig. 6. However the doubly-bent tube may be 



standing, the line join- 



y | - - b| ing the two surfaces 



will always be level. 

 This is of great service 

 to surveyors and such 

 people who must be 

 able to find a level line 

 for the purpose of 

 their observations. 

 The following simple 



experiment is useful in showing that the surface of a liquid at 

 rest is level : 



EXPT. 15. Into a shallow glass vessel pour enough mercury 

 to cover the bottom. Attach a ball of lead to the end of a 

 fine string, and so construct a plumb-line. Hang it over the 

 surface of the mercury, and notice that the 

 line itself and its reflection are in one and 

 the same line. If this were not the case, 

 that is, if the image slanted away from the 

 plumb-line itself, we should know the sur- 

 face of the liquid was not horizontal. 



Liquids communicate Pressure equally in 

 all Directions. It will be desirable first to 

 satisfy ourselves that liquids communicate 

 pressure, and then try to understand the 

 second part of the statement, that they com- 

 municate it equally in all directions. Imagine 

 we have two cylinders in connection, as shown 

 in Fig. 7. Into each cylinder fit a piston with 

 a plate attached to the top, as our illustration 

 also makes clear. If we push one piston down, 

 we notice tha the other moves up. If we put 

 a weight, say 10 Ibs. , on to each piston, they will exactly balance 

 one another, and there is no movement in either case. Each 

 piston is pressed upwards and downwards to an equal degree, 



Fio. 7. To illus- 

 trate Hydrostatic 

 Pressure. The 

 two cylinders are 

 of the same sizej 

 so equal weights 

 upon the pistons 

 balance one an- 

 other. 



