1818.] <f the Profile of Mount Jura. 357 



tion ; without it we can form no idea of the form and relations 

 of the ground. I consider this kind of information as the most 

 important of all. In fact, it is often of less consequence to the 

 engineer to be acquainted with the horizontal distance of an 

 object exactly, than to know its relative height. In all cases, 

 both civil and military, the relation of height is of primary 

 importance. A topography which leaves it out has only the 

 appearance of being useful. 



The difficulty, the tediousness, and the inaccuracy of geome- 

 trical levellings may be objected ; and I admit these objections. 

 Great geodesical levellings formed by the sole practicable 

 system, reciprocal zenith distances taken at different times, are 

 influenced by the anomalies of terrestrial refraction. A good 

 system of previous observations in which all the effects of pertur- 

 bations were combined, would furnish, it is true, a complete 

 suite of circumstantial co-efficients. But this great undertaking 

 is still to commence. Topographical levellings, executed by 

 means of angular data, besides their dependance on horizontal 

 distances and vertical bases, are extremely tedious and delicate. 

 They suppose an exact planimetry ; they are affected by the 

 inappreciable irregularities of retraction. The series of absolute 

 heights thus determined is subject to accumulations of errors, 

 which the probable compensation of opposite errors can but ill 

 obviate. In countries exposed to violent changes, the difficulties, 

 and with them the errors, augment. Very steep declivities often 

 are beyond the limits of the instruments : the horizontal bases, 

 which are less exact in such countries, combined within accurate 

 angles, affect the levels with a degree of uncertainty which may 

 be very considerable. 



The difficulties and imperfections which I have pointed out 

 belong to the most favourable case of all, when the engineer is 

 furnished with proper instruments to give the zenith distances ; 

 and when he possesses a very exact planimetry of the country 

 to be levelled ; and when several absolute heights are already 

 accurately determined ; when he has sufficient leisure, the means 

 of marking out the different signals, &c. Sec. But how far is the 

 solitary geologist, naturalist, or engineer, from this advantage 

 position ! Horizontal bases, the means of measuring them, a 

 sufficient number of known points, exact instruments, and time 

 itself, that fugitive element which presses so much upon the 

 travelling geologist, are here all wanting. Whole ages would 

 not be sufficient to enable an engineer to obtain the complete 

 vertical topography of a country of moderate extent; and yet 

 the geologist and the geographer are often under the necessity 

 of levelling with rapidity, and of determining in a few days the 

 whole profiles of systems of mountains, of which he rarely pos- 

 ses a tolerable chart, and never an exact planimetry. 



Without the assistance of the barometer such a situation 

 would be desperate. This instrument, so simple, so precious, 



