THE ATMOSPHERE. 



of the whole weight exists within four miles of the sur- 

 face of the earth. 



Experiment has proved this weight to be about fifteen 

 pounds on every square inch. If the human body pre- 

 sent a surface of eleven square feet, it must sustain a 

 pressure of upwards of eleven tons. And the pressure 

 upon the whole earth, which exposes about 5,575,680,- 

 000,000,000 square feet, is equal to about 1,164,201,- 

 840,000,000,000 pounds. This seems almost incredi- 

 ble, but we hope to render the fact less startling as we 

 proceed with our illustrations. It may be asked why 

 we are not more sensible of this vast weight. We are 

 so constituted as to require this provision of nature, and 

 from the effect of habit we are insensible to its power. 

 Those who ascend Mont Blanc very sensibly experi- 

 ence a change. They complain of great fulness and 

 distention of the vessels, which arises from the inequa- 

 lity of the external and internal pressures On the 

 surface of the earth, the forces of the internal fluids are 

 counterbalanced by the atmosphere. And it is owing 

 to the equality of these forces that we are insensible to 

 the existence of either. This is further exemplified in 

 the fish, who is sometimes caught at a depth of 2560 feet, 

 where he is compressed by a force equivalent to eighty 

 atmospheres; yet he is not injured, nor are his motions 

 impeded. The internal pressure of his body resists 

 that from without, and therefore he is perfectly protect- 

 ed. Besides, the pressure he supports, acting in all 

 directions, neutralizes itself by promoting as much as 

 it retards his motions. 



Another evidence of the pressure of fluids, is exhib- 

 ited in the following instance. If we place a weight of 

 any kind upon a thin plate of glass, resting by its ex- 

 treme edge, it will be instantly broken through, be- 

 cause all the weight is from above. But if the glass 

 be laid on a flat surface, and the weight then gently 

 placed on it, it will not be broken, because the resis- 

 tance from below is equal to the force from above. If 

 the glass plate were made the bottom of a vessel, and 

 water were poured upon it, it would be destroyed as in 

 the first case But if it were placed in a vessel at any 



