PHILOSOPHICAL TRANSACTIONS. AfQ 



a cubic foot ; whence it follows, that if -^ of a foot of air weighs an ounce, a 

 whole foot will weigh 1-^ ounce. — I suppose, thirdly, that any great vessel may 

 be altogether evacuated of air, or at least of the greatest part of the air, in the 

 following manner. Let any great glass-vessel be taken, that is round and has 

 a neck, and let to the neck be fastened a brass or tin tube, the longer the surer 

 the effect will be. Let there be near the said vessel a stop-cock, so closing the 

 glass that no air can enter. Fill the whole glass, and the whole pipe full of 

 water; then shutting the latter in the extreme part, let the vessel be inverted, 

 so that it stand on its upper part, and let the extreme part of the tube be 

 immersed in water ; and whilst it is immersed in the water, let it be opened, 

 that the water may issue out of the vessel ; which will all go out of it, except 

 part in the tube, and the remaining space above will be empty, there being no 

 way for the air to enter : then shut the neck of the vessel with the stop-cock, 

 and the vessel will be empty. — I suppose, fourthly > it proved that the superficies 

 of spheres increase in a duplicate proportion to their diameters, and their solidity 

 in a triplicate. — I suppose, fifthly, that where one body is lighter in specie than 

 another, the lighter ascends in the other that is heavier, if the heavier be a 

 liquid body ; as a ball of ordinary wood on water, because it is lighter in specie 

 than water ; so also a ball of glass full of air will swim at the top of water, be- 

 cause though glass be heavier than water, yet taking the whole compound of 

 the ball, glass and air together, it is lighter than that, which is only a body 

 of water. 



These things being supposed, it follows, that if we could make a vessel of 

 glass or other matter, that might weigh less than the air that is in it, and 

 should draw out all its air, this vessel would be lighter in specie than air itself, 

 so that it would swim on the top of the air, and ascend on high. — Let the 

 capacity of the vessel be increased more and more, and we shall arrive to such a 

 size, that although it be made of a dense and heavy matter, yet the weight of 

 the air it may contain shall exceed the weight of the matter that makes up the 

 surface of that vessel. — Let us see then of what determinate size a brass vessel 

 may be made; and let us suppose the thinness of the brass to be such, that a 

 plate of it a foot broad and long may weigh 3 ounces; which is not difficult to 

 make. Let us make of such thin plate a round vessel of the diameter of 14 

 feet: then such a vessel will weigh less than the air in it. So that the air being 

 exhausted out of it, and the bare vessel remaining lighter than an equal bulk of 

 air, must needs of itself mount up into the air. For the surface of the globe 

 will be 6l6 square feet of brass-plate, each of which we have supposed to weigh 

 3 ounces, and the whole 1848 ounces, or 154 pounds. But the capacity of the 

 vessel will be 1437t ^^et; and the weight of the whole air contained in that 



