BALLOON ASCENTS. 
409 
observation of either the lower or upper surface of the mercury 
would give the approximate length of the column of mercury. 
The barometer was furnished with its own thermometer, 
whose bulb was immersed in a tube of mercury of the same 
diameter as that of the barometer. The readings of this 
thermometer frequently read from 30° to 40° in excess of those 
of the sensitive thermometers. 
The bulbs of the sensitive thermometers were long and 
cylindrical, being about f inch in length, and -^V inch in 
diameter. The graduations extended downwards to minus 40°, 
and were all on ivory scales. These thermometers, on being 
removed from a room heated 20° degrees above that of an 
adjoining apartment, acquired the temperature within a 
degree in 10 seconds ; but on taking the thermometer back to 
the heated apartment it took nearly double the time to rise to 
within J a degree of the true temperature. They were 
sufficiently sensitive, therefore, for my purposes, and no cor- 
rection ordinarily is needed for sluggishness except only when 
the balloon was moving with great and unchecked rapidity. 
Besides the instruments shown in the Plate, there were 
ozone papers pinned to the table and to the cordage near, a 
compass, magnetical instruments, my note-book, &c. My 
position wms in the front of the table, almost equidistant 
from the extreme right and left instruments. 
The successful working of these instruments depended 
very much, as I have before said, on their arrangement, a 
quick eye, and the orderly habit of a trained observer; the 
arrangements of reading of every instrument, and of every 
subject of investigation were such that the one constantly 
checked the other ; any erroneous reading of the dry-bulb 
thermometer, for instance, was shortly detected by the reading 
of differently graduated, spirit, or gridiron-bulb thermo- 
meters, and any systematic error in the reading of the wet- 
bulb thermometer was checked by the observations of the 
hygrometers, whose readings, though related, were very dif- 
ferent. Thus the arrangements included a system of checks,, 
so that it was not possible to continue erroneous readings, 
when either the instruments needed attention, as the wet- 
bulb requiring more water, or the water not frozen on it when 
it ought to be, or still frozen when it ought not to be (in the 
latter case requiring the immediate application of heat, the 
only source of which in the balloon is the mouth), or from the 
painful state of the observer at times, losing to some extent the 
power of accurate observation. These arrangements were neces- 
sary, the situation of an observer in a balloon being so pecuhar, 
and there being no means afterwards of discovering erroneous 
observations, if not discovered at the time, or the means of 
