[ 237 ] 96 



transient stations are from one another, tbe more the number of observations 

 has to be augmented: and, instead of several hours, it may be necessary to 

 <Jevote several days, several weeks, and even several months, according to 

 circumstances, of which the observer is naturally the judge. In all cases, 

 however, he has it in his power to obtain dilFerencesof level almost as exact 

 as he could procure them trigonometrically. In rugged and mountainous 

 regions, full of accide?its, levelling with the barometer will afford results 

 preferable to those of trigonometric measurement. 



At first sigiit, to determine altitudes by the heights of the barometer, 

 would seem to be a problem easily solved in practice. Those who think it 

 really so, would, I apprehend, upon trial, find themselves deceived. There 

 is, in fict, a crowd of considerations to be reflected upon, of attentious and 

 care to be given, arising from and dependent on the construction of the in- 

 struments employed, and on the circumstances in which an observer finds 

 himself placed — particulars which cannot be foresee!), much less prescribed 

 in advance — that he only can estimate, and without the due estimation of 

 which, observations apparently well made are, in reality, good for nothing. 

 For instance: in the first place, the traveller must be familiar with all parts 

 of his instruments; he must be able to repair them, in case of accidents, if 

 they are reparable. He must, further, be perfectly acquainted with the laws 

 of meteorology relating to these instruments, that, in any movement or va- 

 riation actually noticed, he may distinguish between what is the effect of 

 accidental causes, or what is a reofular and periodic movement. Then, to 

 render the observations made at different stations duly comparable, it is ne- 

 cessary that the barometers and thermometers should have been compared 

 one with another, respectively, both in advance and afterwards, if the last is 

 possible ; and that proper account be taken of their discrepancies in the re- 

 duction of the calculation. I need not speak of the additional corrections 

 which have to be applied when the comparison is made between barometers 

 of different manners of construction — as, for instance, between syphon and 

 cistern barometers ; or between those of the latter class, the level of whose 

 cistern is relative, and those whose cistern is adjustable. Of course, I need 

 hardly mention either the precautions which all and any of these instru- 

 ments require in their transportation. The least negligence in this partic- 

 ular may give rise to permanent regret. Indeed, chronometers and barom- 

 eters are inexhaustible sources of anxiety for the travelling ge^ grapher. I 

 do not know moments of greater annoyance than those when one finds a 

 chronometer stopped, or a baroraete^ broken, just when he is about confi- 

 dently to use them. 



When the period for the calculation has arriv^ed, comes first the dis- 

 cussion of the observations, which are to be compared — a tedious, but 

 indispensable labor. In this the correspondence and simultaneity of 

 readings have to be settled ; care must be taken to compare with one 

 another only those observations made under conditions of the atmosphere 

 as nearly as may be similar; and the circumstances of the two stations, 

 during the corresponding periods, are to be investigated, to see that the 

 two instruments have been affected generally in the same manner. But 

 such an entire accord cannot be hoped for when the question is to deter- 

 mine a difference of level between two points at a great distance apart, 

 from a large number of corresponding observations. For these cases we 

 have to take into consideration, and apply the conclusions which have been 



