"
After this ascent Robertson became acquainted with some savants
of Hamburg, and amongst others with Professor Pfaff, who was
interested in aerial travelling as a means of settling certain
meteorological problems. Some days after Robertson's ascent, the
professor wrote to him--
"You speak of a certain height at which the hydrogen gas will
find itself in equilibrium in the air of the atmosphere. I
believe that this height is the extremity of the atmosphere
itself; for as the gas has an elasticity much greater than that
of the air, it will go on dilating as it mounts into the higher
regions of the atmosphere, and its specific weight will diminish
as the weight of atmospheric air diminishes; and it will not
cease to mount until it rises above the atmosphere itself, if two
conditions be completely fulfilled--1, the condition that the gas
may be allowed to dilate without leaving the balloon as it rises;
2, the condition that the gas shall not be allowed to mix at all
with the atmospheric air."
Another ascent was arranged for the 14th of August, in which
Robertson was to be accompanied by the professor, but the latter,
yielding to the entreaties of his family, did not go. "I went up
with my friend Lhoest," says Robertson, "at forty-two minutes
past twelve midday.
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