MECHANICS. 49 
valve leading to the air-vessel again closes; the conical valve first mentioned 
falls by its own weight; the water commences again to escape through it, 
and the play of the valves, or the butting of the ram, begins afresh. 
E. Stratics or Arrirorm Boptes, or Gases.—AERostTaATIcs. 
Gaseous or aeriform bodies, among which the atmospheric air occupies the 
most important place, in some respects form a great contrast to the true 
liquids. At an earlier period, atmospheric air was considered as a simple body 
—an element; at the present day, however, its component parts are well 
known, and its place among compound bodies ascertained. It shares with the 
other gases, as well as with solid and liquid bodies, the same general peculi- 
arities, and is also subject to the influence of gravitation and of molecular 
forces. 
Atmospheric air surrounds the globe on all sides, having a thickness of 
from 30 to 35 miles ; it is the cause of a great number of phenomena, some 
of which will here be referred to, others belonging to the subject of 
meteorology. 
That the air had weight was known to Aristotle; Galileo, however, and, 
after him, Torricelli, were the first to prove this by experiment. Exhausting 
the air from a hollow globe, suspending this to the end of a balance brought 
into equilibrium by means of weights, and afterwards allowing the air to 
enter the globe, it will be found that equilibrium is again destroyed, and 
must be restored by the imposition of more weights: their amount will 
express the weight of the air contained. 
The molecular force acts in gaseous bodies very differently from what it 
does in the case of liquids and solids, endeavoring to separate the molecules 
one from another, this influence being called elasticity or tension of gases. 
Of the activity of this force we may be convinced by introducing a well- 
closed bladder under the receiver of an air-pump. When a vacuum is 
produced, the contained air expands the bladder as exhaustion proceeds. 
The expansive force of air is unlimited, as in a state of greatest expansion 
it still exerts a pressure upon the containing walls. For this reason gases 
can have no free surface like solids and liquids, as they would extend illi- 
mitably into space; there is, therefore, for them only one condition of 
equilibrium, namely, that the elasticity in one and the same layer is equal, 
For equilibrium, therefore, the lower layers must constantly remain the 
densest ; for which reason the pressure of the atmosphere must be greater at 
the leve] of the sea than on the tops of mountains. It must not be under- 
stood, however, from what has already been said, that as the air can have 
no free surface, the assumption of a limit of the atmosphere to some miles is 
erroneous. This rests upon grounds hereafter to be stated. 
The atmospheric pressure may be measured ; and to its existence Innu- 
merable phenomena testify. Immerse the lower end of an open tube into 
water, the fluid will rise into it, according to the laws of hydrostatics, to an 
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