I 237 3                           96
transient stations are from one another, the more the number of observations
has to be augmented; and, instead of several hours, it may be necessary to
devote several days, several weeks, and even several months, according to
circumstances, of which the observer is naturally the judge. In all cases,
however, he has it in his power to obtain differences of level almost as exact
as he could procure them trigonometrically. In rugged and mountainous
regions, full of accidents, levelling with the barometer will afford results
preferable to those of trigonometric measurement.
At first sight, to determine altitudes by the heights of the barometer,
would seem to be a problem easily solved in practice. Those who think it
really so, would, I apprehend, upon trial, find themselves deceived. There
is, in fact, a crowd of considerations to be reflected upon, of attentions and
care to be given, arising from and dependent on the construction of the in-
struments employed, and on the circumstances in which an observer finds
himself placedparticulars which cannot be foreseen, much less prescribed
in advancethat he only can estimate, and without the due estimation of
which, observations apparently well made are, in reality, good for nothing.
For instance: in the first place,the traveller must be familiar with all parts
of his instruments; he must be able to repair them, in case of accidents, if
they are. reparable. He must, further, be perfectly acquainted with the laws
of meteorology relating to these instruments, that, in any movement or va-
riation actually noticed, he may distinguish between what is the effect of
accidental causes, or what is a regular and periodic movement. Then, to
render the observations made at different stations duly comparable, it is ne-
cessary that the barometers and thermometers should have been compared
one with another, respectively, both in advance and afterwards, if the last is
possible ; and that proper account be taken of their discrepancies in the re-
duction of the calculation. I need not speak of the additional corrections
which have to be applied when the comparison is made between barometers
of different manners of constructionas, for instance, between syphon and
cistern barometers; or between those of the latter class, the level of whose
cistern is relative, and those whose cistern is adjustable. Of course, I need
hardly mention either the precautions which all and any of these instru-
ments require in their transportation. The least negligence in this partic-
ular may give rise to permanent regret. Indeed, chronometers and barom-
eters are inexhaustible sources of anxiety for the travelling geographer. I
do not know moments of greater annoyance than those when one finds a
chronometer stopped, or a barometer broken, just when he is about confi-
dently to use them.
When the period for the calculation has arrived, comes first the dis-
cussion of the observations, which are to be compareda tedious, but
indispensable labor. In this the correspondence and simultaneity of
readings have to be settled ; care must be taken to compare with one
another only those observations made under conditions of the atmosphere
as nearly as maybe similar; and the circumstances of the two stations,
during the corresponding periods, are to be investigated, to see that the
two instruments have been affected generally in the same manner. But
such an entire accord cannot be hoped for when the question is to deter-
mine a difference of level between two points at a great distance apart,
from a large number of corresponding observations. For these cases we
have to take into consideration, and apply the conclusions which have been