LAWS OF FLUIDS. 47 



are required to balance any weight with a mechanical power, 80 

 pounds will be wanted to put the machine in motion.] 13. How does 

 oil lessen friction.? 14. What are the two kinds of friction? 15. 

 How does it appear that in the pulley what is gained in power is lost 

 in time ? 16. Explain the principle of the wedge by fig. 6. 17. Of 

 the screw by fig. 3. 



LESSON 23. 



TJie Lmos of Fluids. 



Hydrostat'ics, a term formed of two Greek words, which signify 

 water, and the science which considers the weight of bodies, 

 viz. statics. 



Gas, all kinds of air differing from the atmosphere are called gas. 



Cu'bical, having six square and equal sides. 



Or'ifice, any opening or perforation. 



A FLUID is a body, the parts of which yield to any impres- 

 sion, and are easily moved among each other. Philosophers 

 have generally imagined that the particles of which fluids 

 are composed must be exceedingly small, because with their 

 best glasses, they have never been able to discern them. 

 And they contend that these particles must be round and 

 smooth, since they are so easily moved among one another. 

 This supposition will account for many circumstances which 

 belong to them. If they are round, there must be vacant 

 spaces between them. If a number of cannon balls were 

 placed in a large vessel so as to fill it up even with the edge ; 

 though it would hold no more of tliese balls, yet a great 

 number of smaller shot might be placed in the vacuities be- 

 tween them : and when the vessel would contain no more 

 small shot, a great quanlity of sand might be shaken in, and 

 between the pores of these, water or other fluids would readily 

 insinuate themselves. In a similar manner, a certain quan- 

 tity of particles of sugar can be taken up in water without 

 increasmg the bulk, and when the water has dissolved the 

 sugar, salt may be dissolved in it, and yet the bulk remain 

 the same. And this is easily accounted for, if we admit the 

 particles of water to be round. 



Fluids are either non-elastic and incompressible, as water, 

 oil, mercury, and others, or elastic and compressible, as air, 

 steam, and the different gases. The science which treats of 



