TOO NATURAL HISTORY OF AQUATIC INSECTS CH. 



solid upwards. Suppose that a solid of the same 

 density as water floats with part of its surface in 

 contact with air, and that weights are gradually added 

 to it. The result will be that the surface of the water 

 around the upper edge of the solid will become more 

 and more depressed. The sides of the depression 

 will take a more vertical position, until at last the 

 upward pull of the film becomes unable to with- 

 stand further increase of weight. If this point 

 is passed, the solid will sink. Before this point 

 is attained, we shall have the solid, though denser 

 than water, kept at the surface by the pull of the 

 surface-film. 



This explanation will be followed more readily 

 with the help of a simple experiment 



Take a float made out of a large cork, weighted by 

 a screw driven into its lower end. To the top of the 

 cork a small glass tube may be attached in a vertical 

 position. The lower end of the tube should be closed 

 and the upper end open. The weight must be 

 adjusted so that the top of the tube is only just out 

 of the water. Then add small weights one by one. 

 Small pieces of sheet metal can be placed by a 

 forceps on the cork. As the weight increases, the 

 tube slowly sinks until it becomes flush with the 

 water. Still no water flows in. We can go on 

 adding weights until the float depresses the surface, 

 as the reflections show, and yet, if the weight is not 

 excessive, the float does not sink. But if we take a 

 loop of thread wetted with water, and draw it across 

 the open mouth of the tube from side to side, the 

 surface-film will be drawn over the hole : then there 



