256 Mr. W.H. Hyett on the Heights of some of the 
eminences in question, offered an opportunity not to be lost of 
having measurements made. 
I therefore proposed to our excellent President to get (as best 
I could) a list of the heights of those hills from which we derive 
our name, and which in the course of our excursions we so fre- 
quently climb ;—a subject of peculiar interest therefore to our- 
selves, and not without importance to all who study the geology, 
botany, &c. of this range. Immediately on receiving his con- 
currence I wrote to Capt. Yolland, R.E., who has the mapping 
department of the Ordnance under his direction, and the com- 
mand of the parties now executing the survey of the Severn. 
Observing that the signal staffs of their present Trigonometrical 
Survey afforded the easy means of taking the vertical as well as 
the horizontal angles, and of acquiring all the information 
which the public needed, I ventured to express a hope to that 
officer that he would afford it. 
In reply he promised to communicate the information re- 
quested, and has since most obligingly supplied the approximate 
heights above the mean level of the sea of sixteen remarkable 
points in our vicinity which I shall presently read to you, together 
with other data which I have myself obtained by the aid of the 
aneroid barometer lately invented in France, and much vaunted 
as applicable to the measurements of heights. 1 then procured 
one of these instruments from Dent, with his pamphlet upon it, 
and will now give the results of its comparison with the mea- 
surements received from Capt. Yolland. 
It may be as well however first to make a few remarks on this 
new instrument, with a view to show how far it may be appli- 
cable in its present state to the purpose of measuring altitudes. 
It is probably known to most of you, that im carrymg a mercu- 
rial barometer to the top of a high mountain, the mercury sinks 
from two causes, the one purely barometric, the other thermo- 
metric. Whilst for every 850 feet of perpendicular ascent the 
weight of the air decreases so as to show a fall, in its counterpoise 
the quicksilver, of about an inch—for every 300 feet of ascent there 
is also a decrease in the temperature of 1° Fahrenheit, occasioning 
a proportional contraction in the quicksilver in the tube, making 
it stand so much lower than it ought to do were its descent due 
to the diminished pressure of the air alone. To calculate there- 
fore correctly the height indicated by the mercurial barometer, 
allowance is always made for decreasing temperature, and tables 
have been compiled for this purpose from the known rate at 
which mercury contracts by cold. 
The same double effect is doubtless produced in the aneroid 
barometer, which Mr. Dent says 1s compensated by means of gas 
