a shock. The weight,
amounting to thirty pounds, reached the ground before us, and the
balloon, thus lightened, came softly to the ground between Wichtenbech
and Hanover, after having run twenty-five leagues in five and a half
hours."

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. In a minute or
two we rose up between two masses of cloud, which seemed to open up and
offer us a passage. The upper surfaces of these clouds are not uniformly
level, like the under sides seen from the earth, but they are of a
conical or pyramidal shape. These imposing masses seem to precipitate
themselves upon the earth, as if to engulf it, but this optical illusion
was due to the apparent immobility of the balloon, which at the moment
was rising at the rate of about twenty feet per second.

"The fear of losing the view of the Baltic, which we perceived between
the clouds at intervals, obliged us to renounce the project of rising as
high as on the last occasion. The barometer was at fifteen inches, and
the thermometer one degree below zero, when I let off two pigeons.

"One descended in a diagonal direction, its wings half open but not
moving, with a swiftness which seemed that of a fall. The other flew
for an instant, and the