24 



HYDROSTATICS. 



through the ordinary vent, comes dur- 

 ing the intervals between the empty- 

 ing and filling of the hollow. If the 

 cavern is fed with water by the runners 

 E, E, E, (Jig. 24.) and these, from the 

 drought, are not sufficient to raise its 

 level as high as I K, the syphon out- 

 let F B cannot act at all*; but the 

 stream H will flow constantly. If 

 the runners, during the wet weather, 

 fill the cavern up to the level I K, the 

 syphon outlet will act, and drain off the 

 water, which may escape by various 

 small rills at B ; if this outlet begins 

 below the level of the common out- 

 let, supposing it to be at H', instead of 

 H, it will carry off the water, while 

 none can escape through H ; and if it 

 carries off as much as the runners E, 

 E, E, pour in, the water will not rise 

 as high as H', and none will flow 

 through it, until the cavern is drained, 

 and it is filled again as high as the 

 point H'. 



CHAPTER VIII. 

 Capillary Attraction. 



HITHERTO we have seen no exception 

 to the general rule, that all the parts of 

 a liquid stand always at the same 

 height if left to themselves, and that 

 consequently no liquid can of itself 

 rise higher in the inside of a tube, than 

 it stands on the outside. But there is 

 an exception, or rather an apparent ex- 

 ception, to this rule, which must now 

 be explained. 



If a drop of water, or any liquid of a 

 like degree of fluidity, be pressed upon 

 a solid surface, it will wet that surface, 

 and stick to it, instead of keeping toge- 

 ther, and running off when the surface 

 is held sloping. This shows that the 

 parts of the liquid are more attracted 

 by the parts of the solid than by one 

 another. In the same manner, if you 

 observe the edge of any liquid in a 

 vessel, as wine in a glass, and note 

 where it touches the glass, you will see 

 that it is not quite level close to the glass, 

 but becomes somew r hat hollow, and is 

 raised up on it, so as to stand a little 

 higher at the edge than in the middle 

 and other parts of its surface. It ap- 

 pears, therefore, that there is an attrac- 

 tion at very small distances from the 

 edge, sufficient to suspend the part of 

 the fluid near it, and prevent it from 

 sinking to the level of the rest. Sup- 

 pose the wine-glass to be diminished 



so as to leave no room for any of the 

 wine in the middle, which lies "flat and 

 level, but only to leave room for the 

 small rim of liquor raised up all round 

 on the side of the glass ; in other words, 

 suppose a very small tube, placed with 

 its lower end just so as to touch the 

 liquor ; it is evident that the liquor will 

 stand up somewhat higher in the tube 

 than on the outside ; and if the tube 

 be made smaller and smaller, the 

 liquor will rise higher, there being 

 always less weight of liquid to keep 

 it from rising and counterbalance the 

 attraction of the glass. 



Tubes of this very small bore are 

 called Capillary, from a Latin word, 

 signifying hair, because they are small 

 like hairs. Generally, any tube of 

 less than the twentieth of an inch dia- 

 meter in the inside is called a capillary 

 tube; and if it is placed so as to 

 touch the surface of water, the wa- 

 ter will rise in it to a height which is 

 greater the smaller the bore of the tube 

 is. If the diameter of the tube is the 

 fiftieth part of an inch, the water will 

 rise to the height of one inch ; if it be 

 the hundredth part of an inch, the wa- 

 ter will rise two inches ; if the two 

 hundredth part of an inch, the water 

 will rise four inches, and so on in pro- 

 portion as the bore is lessened. Now 

 the quantity of water raised in these 

 tubes is in proportion to the square of 

 the diameter, multiplied by the height 

 it rises to, because cylinders are to one 

 another as the squares of their diame- 

 ter multiplied by their lengths ; there- 

 fore, the height "being inversely as the 

 diameter, it follows, that the quantity 

 of water raised is in proportion to the 

 diameter ; and the circumferences of 

 the tubes being also in the proportion of 

 the diameter, it is plain that the quan- 

 tity of water raised is in proportion to 

 the circumference of the tube, or the 

 quantity of matter in the ring of the 

 tube which first touches the water. 

 From hence arises a probability that 

 this effect is produced by the attrac- 

 tion of the ring of the tube. But the 

 subject is involved in considerable ob- 

 scurity ; and although philosophers have 

 thrown much light upon it by mathe- 

 matical reasoning, great doubt remains 

 respecting the explanation of the fact. 

 Some hold that the water is raised and 

 supported by the attraction of the ring 

 of glass immediately above the water's 

 surface ; but then the ring immediately 



