PRESSURE OP FLUIDS. 49 



out of it. In a cubical vessel the pressure downwards will 

 be double the lateral pressure on one side ; for every particle 

 at the bottom of the vessel is pressed upon by a column of 

 the whole depth of the fluid, whilst the lateral pressure 

 diminishes from the bottom upwards to the surface, where 

 the particles have no pressure. The upward pressure of 

 fluids may be shown by a machine, called the hydrostatic 

 bellows. It consists of two oval or round boards, covered 

 with leather so as to rise and fall like the common bellows, 

 but without valves. A long tube is fixed to the upper board 

 and weights placed upon it. When the tube is supplied with 

 water, it will, by its upward pressure, sustain and lift up the 

 weights. The pressure of water and other fluids differs from 

 the gravity or weight, in this respect ; the weight is accord- 

 ing to the quantity ; but the pressure is according to the 

 Ijerpendicular height. Dr. Goldsmith relates that he once 

 saw a strong hogshead split by the following experiment. A 

 strong small tube, made of tin, about twenty feet long, was 

 cemented into it, and then water was poured in to fill the 

 cask ; when it was full and the water had risen nearly to the 

 top of the tube, the vessel burst with a prodigious force. 

 This extraordinary power may be greatly increased by a 

 forcing piston placed in the tube. A similar method has been 

 adopted in forming a machine, called a hydrostatic press, by 

 which hay or cotton may be brought into a compass twenty 

 or thirty times less than it usually occupies, 



Questions. — 1. What is a fluid? 2. What have philosophers 

 generally imagined respecting the particles of fluids ? 3. What il- 

 lustration is given respecting the vacant spaces between the parti- 

 cles of fluids ? 4. What are the two kinds of fluids ? 5. Define 

 Hydrostatics. 6. Why are the non-elastic fluids said to be incompres- 

 sible .' 7. Describe the experiment made at Florence. 8., What was 

 concludod at the time, and what have later experiments shown .? 9. 

 What is the difference between the gravity of fluids and solids .' 10. 

 Why does water always find its level ^ 11. What is said of the direc- 

 tion in which fluids press ? 12. What is the cause of the downward 

 and lateral pressure .■* 13. What is said of the lateral pressure in a 

 cubical vessel .' H. How may the downward pregsure be shown i* 

 1.5. How does the pressure of a fluid differ from the weight*' 16. 

 What is related by Dr. Goldsmith .? 17. What is said of the hydro- 

 static press ? 18. Illustrate the pressure of fluids by figures 25. 24. 

 and 23. 19 What is said of what is called the hydrostatical paradox .' 

 5 



