W. LE CONTE STEVENS. on 
of the rigidity required to prevent it from breaking 
under the weight of the contained water if full, or col- 
Japsing under the pressure of the surrounding air if 
vacuous. His experiment was not actually tried, but 
the only valid objection to its probable success that he 
expressed was ‘‘that the Almighty would never allow 
an invention to succeed by means of which civil govern- 
ment could be so easily disturbed.”’ 
Several unsuccessful attempts were made about the 
same time to fly by the use of artificial wings, or to sail 
through the air by attaching to the body a frame-work 
covered with canvas, like an open umbrella. The fu- 
tility of all attempts to fly was theoretically demon- 
strated by Borelli, an Italian physicist, whose posthu- 
mous work, De Motu Animalium, was published at 
Rome in 1680. His reasoning was based upon an exam- 
ination of the muscular adaptation of birds to flight, 
and a consideration of the mechanical energy implied. 
He showed that the muscles and bony framework of the 
human body were far from adequate to do for it the 
same kind of lifting work that is accomplished by the 
bird, and hence that athletic training enough for pur- 
poses of flight was hopeless. 
In 1766 Cavendish and Watt independently discov- 
ered the gas hydrogen, the most remarkable property of 
which was its low specific gravity. Dr. Black, of Edin- 
burgh, attempted to utilize it for the purpose of causing 
a gas bag to ascend, but failed to secure a bag that was 
light enough. Cavallo, in 1782, caused soap bubbles to 
ascend by filling them with hydrogen. During the 
latter part of the same year, at Avignon, two paper 
manutacturers, the brothers Montgolfier, attempted 
Similar experiments with hydrogen, using large paper 
bags as receptacles. They failed on account of inability 
to confine a gas of such high diffusive power within 
bags of such porous material as paper. Noticing that 
