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 in carrying 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 is compensated by means of gas 



