APPENDIX. 465 



staff; with results even more true than those of the 

 instruments to which I have objected : though every 

 geologist ought to know, that nothing is gained hy 

 this fantastical accuracy. Where rocks are inacces- 

 sible, a pocket telescope will often be of use ; but let 

 no one imagine that he can thus supersede the neces- 

 sity of a manual examination. 



If it has been the fashion to record the altitudes of 

 hills, this is rather a question of physical geography. 

 Yet they who have this ambition, must submit to the 

 trouble of a barometer ; careful, however, that they 

 do not mislead themselves, in the use of that which 

 requires many minute attentions ; the most frequent 

 of its errors arising from the gradual admission of air, 

 as the fault is also not discoverable till correction is 

 impossible. The pressure of the air itself is under- 

 going frequent, and, often, sudden and considerable 

 alterations ; so that imaginary differences of altitude 

 are sometimes inferred, from this cause, acting during 

 the intervals of observation, or, under the use of dis- 

 tant instruments, or, still more, under travelling ob- 

 servations referred to a remote barometer. By com- 

 paring the registers of Gordon castle, Kinfauns, Mr. 

 Playfair, and Greenwich, all carefully kept, I long ago 

 proved, in a paper which the Geol. Soc. did not print, 

 that under a varying state of the atmosphere, the 

 changes are very far from simultaneous in different 

 places. 



It is barely sufficient to name such things as a 

 measuring tape and a chisel ; while, for blasting, the 

 geologist must trust to occasional assistance. But in 

 spite of inexperience or indolence, let him be assured 

 that he can decide on no rock on which he has not 

 laid his hammer. Their physiognomies are uncertain, 

 they are often obscured by decomposition or by lichens, 



VOL. n. H H 



