30 



FORCE AND ENERGY 



FIG. 24. 



A Needle Floating on 

 Water. 



up at the edges, and the surface curves inwards, or is concave ("hol- 

 lowed out"). 



In a greased vessel water has a convex upper surface, like that of 

 mercury in a glass vessel. The cohesion of water is evidently stronger 

 than the adhesion between water and grease. 



The curved surface of a liquid is called a meniscus. In 

 reading the level of water in a graduated cylinder (Fig. 5) 

 we read at the bottom of the meniscus. 

 That the surface of a liquid acts as 

 though it were an elastic covering, and 

 that it can be stretched, is shown by an 

 experiment in which a needle is floated 

 upon water (Fig. 24). The needle must 

 be put down carefully, or it will break 

 the elastic surface of the water. The experiment suc- 

 ceeds better if the needle is 

 slightly greased, so that the water 

 is certain not to wet it. 



32. Capillary Action. If you 

 leave one end of a towel in a 

 bowl of water, the water rises, 

 against gravity, into the towel. 

 If you let a wet string, or, bet- 

 ter, a wet strip of cloth, hang 

 over the side of a dish of water 

 (Fig. 25), the water will rise 



through the string or cloth, and so flow out of the dish. 

 If you touch a drop of ink with a blotter, the whole drop 

 will flow into the blotter. What causes this phenomenon? 

 We say that the force exerted in these cases is " capillary " 



FIG. 25. 



Water Flowing by Capillary Actiop 

 Over the Side of a Dish. 



