MENISCUSES. 63 



from the opening it suffices to pour some water into the tube up to the level 

 of the water outside (fig. 64). The outer pressure exercised on the disc, 

 as well as the pressure beneath, is now equal to the weight of a body of 

 water having for its base the surface of the opening of the tube, its depth 

 being the distance from the cardboard to the level of the water. 



Syringes, pumps, etc., are the effects of atmospheric pressure. Balloons 

 rise in the air by means of the pressure of gas ; a balloon being a body 

 plunged in gas, is consequently submitted to the same laws as a body plunged 

 in water. 



Boats float because of the pressure of liquid, and water spurts from a 



Fig 65. Experiment on the convexity of a meniscus. 



fountain for the same reason. I recollect having read a very useful applica- 

 tion of the principles of fluid pressure. 



A horse was laden with two tubs for carrying a supply of water, and 

 in the bottom of the tubs a valve was fixed. When the horse entered the 

 stream the tubs were partly immersed; the water then exercised its upward 

 pressure, the valve opened, and the tubs slowly filled. When they were nearly 

 full the horse turned round and came out of the water ; the pressure had 

 ceased. 



Thus the action of the water first opened the valve, and then closed it. 



The particular phenomena observable in the water level in narrow spaces, 

 as of a fine glass tube, or the level of two adjoining waves, capillary 



