be entirely exhausted of
nearly all the air contained therein. Then he takes Euclid's proposition
to the effect that the superficial area of globes increases in the
proportion of the square of the diameter, whilst the volume increases in
the proportion of the cube of the same diameter, and he considers that
if one only constructs the globe of thin metal, of sufficient size, and
exhausts the air in the manner that he suggests, such a globe will be so
far lighter than the surrounding atmosphere that it will not only
rise, but will be capable of lifting weights. Here is Lana's own way of
putting it:--

'But so that it may be enabled to raise heavier weights and to lift
men in the air, let us take double the quantity of copper, 1,232 square
feet, equal to 308 lbs. of copper; with this double quantity of copper
we could construct a vessel of not only double the capacity, but of
four times the capacity of the first, for the reason shown by my fourth
supposition. Consequently the air contained in such a vessel will be 718
lbs. 4 2/3 ounces, so that if the air be drawn out of the vessel it
will be 410 lbs. 4 2/3 ounces lighter than the same volume of air, and,
consequently, will be enabled to lift three men, or at least two, should
they weigh more than eight pesi each. It is thus manifest that the
larger the ball or vessel is made, the thicker and more solid can the
sheets of copper be made, because, although the weight will increase,
the capacity of the vessel will increase to a greater extent and with it
the weight of the air therein, so that it will always be capable to lift
a heavier weight. From this it can be easily seen how it is possible to
construct a machine which, fashioned like unto a ship, will float on the
air.'

With four globes of these dimensions Lana proposed to make an aerial
ship of the fashion shown in his quaint illustration. He is careful to
point out a method by which the supporting globes for the aerial ship
may be entirely emptied of air; (this is to be done by connecting to each
globe a tube of copper which is 'at least a length of 47 modern Roman
palm).' A small tap is to close this tube at the end nearest the globe,
and then vessel and tube are to be filled with water, after which the
tube is to be immersed in water and the tap opened, allowing the water
to run out of the vessel, while no air enters. The tap is then closed
before the lower end of the tube is removed from the water, leaving no
air