“At this elevation, the glass, the brimstone, and the Spanish wax were not electrified in a manner to show any signs under friction—at least, I obtained no electricity from the conductors or the electrometer.
“I had in my car a voltaic pile, consisting of sixty couples—silver and zinc. It worked very well on the occasion of our departure from the earth, and gave, without the condenser, one degree to the electrometer. At our great elevation, the pile gave only five-sixths of a degree to the same electrometer. The galvanic flame seemed more active at this elevation than on the earth.
“I took two birds with me on coming into the balloon—one of these was now dead, the other appeared stupefied. After having placed it upon the brink of the gondola, I tried to frighten it to make it take to flight. It moved its wings, but did not leave the spot; then I left it to itself, and it fell perpendicularly and with great rapidity. Birds are certainly not able to maintain themselves at such elevations.
“It is notable that the atmosphere, which was of a perfect purity near the earth, was grey and misty above our heads, and the beautiful blue sky seen from the surface did not exist for us, although the weather was calm and serene, and the day the most beautiful that could be. The sun did not seem dazzling to us, and its heat was diminished owing to our elevation.
“At half-past eleven, the balloon was no longer visible from Hamburg. The heavens were so pure beneath us that everything was distinctly seen by us, though very much diminished by distance. At 11.40, the town of Hamburg seemed only a red point in our eyes; the Elbe looked like a straight ribbon. I wished to make use of an opera-glass, but what surprised me was that when I lifted it up it was so cold that I had to wrap my handkerchief around it to enable me to hold it.
“Not being able to support our position any longer, we descended, after having used up much gas and ballast. Our descent caused that degree of terror among the inhabitants which the size of our balloon was calculated to inspire in a country where such machines had never before been seen. We descended above a poor village called Radenburg, a place amid the heaths of Hanover. Our appearance caused great alarm, and even the beasts of the field fled from us.
“While our balloon rapidly approached the earth, we waved our hats and flags, and shouted to the inhabitants, but our voices only increased their terror. The villagers rushed away with cries of terror, leaving their herds, whose bellowings increased the general alarm. When the balloon touched the ground, every man had shut himself up in his own house. Having appealed in vain, and fearing that the villagers might do us some injury, we resolved to re-ascend.
“In making this second ascent, we threw over all our ballast; but in this we were imprudent, for after sailing about at a great height, and having lost much gas, I perceived that our descent would be very rapid, and to provide against accident, I gathered together all the instruments, the bread, the ropes, and even such money as we had with us, and placed them in three sacks, to which I attached a rope of a hundred feet in length. This precaution saved us 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.”