58 THE PLANTS OF THE GRAMPIANS. 
between the notes taken at the starting place, before and 
after the ascent of the hill, would usually show whether any 
and what variation had meantime occurred in the pressure of 
the atmosphere, and in calculating the heights of the resting 
places, an approximate allowance could be made for any 
such variation according to time. I say zz usually," because 
the pressure might have changed in the interval, and have 
again returned to the same figure by the hour when the 
second notes were taken at the inn; and in any such case no 
correction or allowance could be made, because the variation 
would not appear. 
It should be mentioned, that the stoppages were ordinarily 
made at the first station for any shrub or other plant, whose 
exact limits I was more particularly desirous to ascertain ; and 
the heights calculated for the resting places were conse- 
quently the heights at which such plants were first seen. 
The heights of the other plants observed between any two 
resting places, were estimated accordingly as their names in 
the lists appeared near the one or the other of the two places. - 
It wil be obvious from this explanation, that the heights of 
several species were not very precisely ascertained; and the 
heights of all are liable to that want of exactness which be- 
longs to every calculation of altitude founded upon a datum 
so variable as the pressure of the atmosphere. But having | 
in various instances had the opportunity of comparing the 
results of my own observations with those of other persons 
using different methods, I do not entertain any doubt that 
they are sufficiently near approximations to truth, for the 
purposes of botanical geography, where extreme accuracy ÍS 
never required. 
It is scarcely needful to add, that the heights calculated 
from such observations can be only the heights above the 
starting place. To determine the true heights above the level 
of the sea, that of the starting place must also be known: 
and added to the others; and the difficulty of ascertaining | 
that one height is often an impediment in the proceedings 9t 
the botanist, since any inexactness on this point is unayoid- 
