TRUE VOLCANOES. 429 



Pasto and Cumbal (according to specimens collected by Bous- 



I had made use of fresh-filled Torricellian tubes." The apparatus 

 with several tubes ought never to be used in the open air, more espe- 

 ciallv on the summit of a mountain. It is one of those means which, 

 from the conveniences furnished by large towns, may be employed at 

 long intervals, when the operator feels anxious as to the state of his 

 barometer. For my own part, I have had recourse to it only on very 

 rare occasions, but I would nevertheless still recommend it to travel- 

 ers, accompanied by a comparison with the boiling point, as warmly 

 as I did in my Observations Astro7iomiques (vol. i., p. 363-373): "As 

 it is better not to observe at all than to make bad observations, we 

 ought to be less afraid of breakingthe barometer than of putting it 

 out of order. M. Bonpland and I having four different times trav- 

 ersed the Cordilleras of the Andes, the determinations which chief- 

 ly interested us were repeated at different times, as we returned to 

 the places which seemed doubtful. AVe occasionally employed the 

 apparatus qfMulis, in which Torricelli's primary experiment is per- 

 formed, by applying successively three or four strongly-heated tubes, 

 filled with mercury recently boiled in a stone-ware crucible. When 

 there is no possibility of replacing the tubes, it is perhaps prudent not 

 to boil the mercury in the tubes themselves. In this way I have found, 

 in experiments made in conjunction with Lindner, Professor of Chem- 

 istry at the School of Mines in Mexico, the height of the column of 

 mercury in six tubes, as follows : 



259-7 lines (old Paris foot) 250-9 lines (old Paris foot) 

 259-5 2G0-0 



259-9 259-9 



"The two last tubes alone had, by means of heat, been deprived of air 

 by Bellardoni, the instrument maker at Mexico. As the exactness of 

 the experiment depends partly on the perfect cleanliness of the inside 

 of the empty tubes, which are so easily carried, it is a good plan to 

 seal them hermetically over a lamp." As the angles of altitude can 

 not, in mountainous districts be taken from the sea-shore, and the 

 trigonometrical measurements are of a mixed nature and to a consid- 

 erable extent (frequently as much as i or 1-2-7 of the whole height) 

 barometrical, the determination of the height of the elevated plain in 

 which the base line may be measured is of great importance. As cor- 

 responding baromatrical observations at sea are seldom obtained, or 

 for the most part only at too great a distance, travelers are too often 

 induced to take the results they have obtained from a few days' observa- 

 tions, conducted by them at different seasons of the year, as the mean 

 height of the pressure of the atmosphere on the elevated plain and at 

 the sea-shore. " In wishing to know whether a measurement made 

 by means of the barometer possesses the exactness of trigonometrical 

 operations, it is only necessary to ascertain whether, in a given case, 

 the two kinds of measurement have been taken under equally favor- 

 able circumstances, that is to say, by fulfilling those conditions which 

 both theory and long experience have prescribed. The mathematical 

 experimenter dreads the effect of terrestrial refraction, while the phys- 

 ical experimenter has reason to fear the unequal and far from simul- 

 taneous distribution of the temperature in the column of air at the 

 extremities of which the two barometers are placed. It is probable 



