141 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 13-L 



Fig. 135. 



Thus ia two tubes, one of which is double 

 the diameter of the other, the fluid will rise 

 to twice tiie height in the small tube that 

 it will in the larger. The tmth of this 

 principle can be made evident by the fol- 

 lowing beautiful and simple experiment. 

 Two square pieces of plate-glass, C and B, 

 Fig. 135, are arranged so that their sur- 

 faces form a minute angle at A. This po- 

 sition may be easily given them by fasten- 

 ing with wax or cement. When the ends of 



t'.ie plates are placed iu the water, as shown in 



the figure, the water rises in the space between 



them, forming the curve, which is called an 



hyperbola. The elevation of the water between 



the two surfaces will be the greatest at the 



points where the distance between the plates is 



tlie least. 



327. The figure of the surface which bounds 



a liquid in a capillary tube wiU depend upon 



the extent of the attraction which exists between 



the particles of the hquid and the surface of 



the tube. Thus, a column of water contained in a glass capillary tube will 



have a conca\*e fonn of surface, as in Fig. 136, since the 



attraction of glass for water exceeds the attraction of the 



particles of water for each other ; a surface of mercury, on 



the contrary, iu a similar tube, will be convex, see Fig. 



137, since the attraction of glass for mercury is less than 



the mutual attraction of the particles of mercury. 



328. In a capillary tube a 

 liquid will ascend above its 

 general level, wben it wets the 

 tube ; and is depressed below its level wheq 

 it does not wet it. 



329. If the surface of a body repels a liquid, such a body, 

 though heavier, bulk for bulk, than the hquid, may, under 

 some circumstances, float upon it ; and so present an apparen; 

 exception to the general hydrostatic law by which sohd^ 

 ■which are heavier than liquids, bulk for bulk, will sink in them. An exam- 

 ple of this may be shown by slightly greasing a fine sewing-needle, and then 

 placing it carefully in the du-ection of its length upon the surface of water. 

 The needle, although heavier, bulk for bulk, than water, wiU float 



The power of certain insects to walk upon the surface of water without 

 sinkine. has been explained upon the same principle. The feet of these in- 

 sects, like the greased needle, have a capillary repulsion for tho water, and 



Fig. 136. Fig. \Z 



When will a 

 liquid be ele- 

 vated and when 

 depressed in a 

 capillary tu.be 1 



How may a 

 needle be made 

 to float upoQ 

 water ? 



