ronaut observer.
Up to 12,000 feet the temperature had decreased consistently from 82
degrees to 47 degrees, after which it increased 6 degrees in the next
2,000 feet. This by no means uncommon experience shall be presently
discussed. The balloon was now steadily manoeuvred up to 18,636 feet, at
which height freezing point was practically reached. Then with a further
climb 20,000 feet is recorded, at which altitude the ardent philosopher
could still attend to his magnetic observations, nor is his arduous and
unassisted task abandoned here, but with marvellous pertinacity he yet
struggled upwards till a height of no less than 23,000 feet is recorded,
and the thermometer had sunk to 14 degrees F. Four miles and a quarter
above the level of the sea, reached by a solitary aerial explorer, whose
legitimate training lay apart from aeronautics, and whose main care
was the observation of the philosophical instruments he carried! The
achievement of this French savant makes a brilliant record in the early
pages of our history.

It is not surprising that Lussac should own to having felt no
inconsiderable personal discomfort before his venture was over. In spite
of warm clothing he suffered greatly from cold and benumbed fingers,
not less also from laboured breathing and a quickened pulse; headache
supervened, and his throat became parched and unable to swallow food. In
spite of all, he conducted the descent with the utmost skill, climbing
down quietly and gradually till he alighted with gentle ease at St.
Gourgen, near Rouen. It may be mentioned here that the analysis of the
samples of air which he had brought down proved them to contain the
normal proportion of oxygen, and to be essentially identical, as tested
in the laboratory, with the free air secured at the surface of the
earth.

The sudden and apparently unaccountable variation in temperature
recorded by Lussac is a striking revelation to an aerial observer, and
becomes yet more marked when more sensitive instruments are used than
those which were taken up on the occasion just related. It will be
recorded in a future chapter how more suitable instruments came in
course of time to be devised. It is only necessary to point out at this
stage that instruments which lack due sensibility will unavoidably read
too high in ascents, and too low in descents where, according to the
general law, the air is found to grow constantly colder with elevation
above the earth's surface. It i