128 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 107. 



■ 



\n 



As thus constructed, the hydraulic press constitutes the most powerful 

 niechauical engine with which we are acquainted, the limits to its power 

 being bounded only by the strength of the machinery and material. By 

 means of this press, cotton is pressed into bales, ships are raised from the 

 water for repair, chain-cables are tested, etc. etc. 



Will liquids 293. As liquids transmit pressure equally in 

 ar^^riP'^as ^^1 directions, it follows that any given portion 

 downward? ^f ^ liquid Contained in a vessel will press up- 

 v^ard upon the particles above it, as powerfully as it 

 2Jresses downward upon the particles below it. 



_ . This fact may bo illustrated by means of 



How IS the up- , ^ "^ , , . T-,- ■. r.^ re 



ward pressure the apparatus represented m I ig. 1 07. ii 



of lianidK shown ^ pjj^te of metal, B, be held against the bot- 

 uy experiment f ^ ' ° 



torn of a glass tube, g, by means of a stnng, 



V, and immersed in a vessel of water, the water being up to 



the level n n, the plate B will be sustained in its place by the 



upward pressure of the water ; to show that this is the case, ^ 



it is only necessary to pour water into the tube g, until it 



rises to the level n n, when the plate will immediately fall, 



the upward pressure below the plate B being neutralized 



by the downward pressure of the water in the tube g. 



" Some persons find it difiQcult to understand why there 



should be an upward pressure in a mass of liquid, as well 



as a downward and lateral pressure. But if in a mass of 



liquid the particles below had not a tendency upward equal 



to the weight, or downward pressure of the particles of liquid above them, 



they could not support that part of the liquid which rests upon them. Their 



tendency upward is owing to the pressure around them from which they are 



trying to escape."* 



294. The pressure exerted by a 

 column of liquid is proportioned to, 

 or measured by the height of the 

 column, and not by its bulk, or 

 quantity. 



If we take a tube in the form of the letter U, with one of its 

 branches much smaller than t'.e other, as in Fig. 108, and pour 

 •water into one of the branches, we shall find that the liquid 

 will stand at the same height in both tubes. The great mass 

 of liquid contained in the large tube. A, exerts no more press- 

 ure on the liquid contained in the small tube, D, than would a 

 smaller mass contained in a tube of the same dimensions as D. 

 And if A contained 10,000 times the quantity of water that D 

 contained, the water would rise to no greater elevation in D 



than in A. 



• Arnott. 





To what is the 

 pressure of a 

 column of liq- 

 uid propor- 

 tional ? 



Fig. 108. 



