THE ATMOSPHERE. 59 



in the weight of the air at various heights, and which 

 established its pressure. 



If the atmosphere support a column of quicksilver at 

 thirty inches at the level of the sea, we must infer that 

 the height of the fluid will diminish as we ascend. We 

 accordingly find, by a rough calculation, that an ascent of 

 a thousand feet causes the quicksilver to sink one inch. 

 On Mont Blanc it falls to about fifteen inches, show- 

 ing an elevation of fifteen thousand feet. And in De 

 Luc's famous balloon ascent, it sunk to twelve inches, 

 proving an altitude of twentyone thousand feet, the 

 greatest height to which man has ever risen. 



THE SYPHON. 



The name of this instrument is also derived from the 

 Greek, and signifies simply a tube. The syphon is 

 a crooked tube, one leg or branch of which is longer 

 than the other. It is used for raising fluids, emptying 

 vessels, and various hydrostatical experiments. 



The principle of the formation of the syphon, forbids 

 a greater height than thirtytwo feet, when the two ex- 

 tremities rest on a horizontal plane. If now the lesser 

 limb be immersed in water, and the air be removed from 

 the tube, the pressure of the air on the surrounding 

 water will force it to pass off through this instrument. 



From small syphons, the air may be removed in three 

 ways. 1st. By drawing it out by the mouth ; 2d. By 

 a small pump connected with the tube; and 3d. By in- 

 verting it, and filling both limbs with the fluid. Then, 

 on immersing the extremity of the short limb, as just de- 

 scribed, its purpose will be effected. The explanation 

 of the effect may be understood, by recollecting that 

 each extremity is equally pressed by the surrounding 

 atmosphere ; ' but the air not being able to sustain all 

 the water in the longer leg, and being more than able 

 to sustain that in the shorter leg ; Avith the excess of 

 force, therefore, it will raise new water into the shorter 

 leg ; and this new water cannot make its way but by 

 protruding the tirst before it ; by this means is the water 

 continually driven out at the longer leg, as it is con- 

 tinually raised by the shorter.' 



