HYDROSTATICS. 133 



pressure on the side ; for in this instance the surface sustains the •weight of a 

 column equal in height to the whole depth, while the column of pressure upon 

 the side is only equivalent to one half the depth. 



How docs the 300. The actual pressure produced upon 



^>en"quantity the bottoiu aud sldcs of a vessel which con- 

 pL'^wUh^us tains a liquid, is always greater than the 

 weight? weight of the liquid. 



In a cubical vessel, for example, the pressure upon the bottom will be 

 equal to the weight of the liquid, and the pressure on each of the four sides 

 will be equal to one half the weight ; consequently the whole pressure on the 

 bottom and sides will be equal to three times the weight of the liquid. 



inwhatcondi- 301. The surfacc of a liquid when at rest is 



tion is the sur- , _-_ _ 



face of a liquid alwajS HORIZONTAL, Of LeVEL. 



The particles of a hquid having perfect freedom of motion 

 wirface'of aiu among themselves, and all being equally attracted by gravita- 

 quid at rest tion, the whole body of liquid will tend to arrange itself in 



such a manner that all the parts of its surface shall be equally 

 distant from the earth's center, which is the center of attraction. 

 _^ t ■ th "^ perfectly level surface really means one in which every 



true definition part of the surface is equally near the center of the earth ; it 

 Burface?**'^'*^^' must be, therefore, m fact, a spherical surface. But so largo 



is the sphere of which such a surface forms a part, that in 

 reservoirs and receptacles of water of limited extent, its sphericity can not be 

 noticed, and it may be considered as a perfect plane and level ; but when the 

 surface of water is of great extent, as in the case of the ocean, it exhibits this 

 rounded form, conforming to the figure of the earth, most perfectly.* This 

 sphericity of the surface of the ocean is illustrated by the fact, that the masts 

 of a sliip appproaching us at sea, are visible long before the hull of the 

 fjQ ii>j vessel can be seen. In Fig. 



B 'fesei ^^^ ^^^y ^^^^ P^^ °^ *^^ 



J^%^''^^^J^^^Sf!^^^^^^^^ ship above the line A C can 



^^^^f "^~^^^"^^^^^^^^ ^^ ^^^^ hy the spectator at 



>^^^^^^^^^^^^ '^^^k. -^ because the rest of the 



~ ^^^ vessel is hidden by the swell 



of the curve cf the surface of the ocean, or rather of the earth, D E. 



In what man. 302. Water, or other liquids will always rise 

 "id rise "in^^a to au cxact Icvel in any series of different 



scries of tubes , •. • , -i i • , • 



orvesseiscom- tubps, jjipcs, or othcr vcsscls communicating 



municatinf; . ,-> i ,i 



with each other? With eacu othcr. 



• A hoop Burrounditi!; the earth would bend from a perfectly strnight linceishtinehes 

 In a mile. Consequently, if a segnient of the surface of the earth, a mile long were 

 cut off, and laid on a perfect plane, the center of the segment would he only four Inches 

 hiirher than the edges. A small portion of it, therefore, for all ordii ary purposes, may 

 be considered as a perfect plane. 



