97 [ 237 ] 



recommended by the most eminent observers. For example : Mr. Ramond 

 has furnished ns with indications in this regard, derived from his long ex- 

 perience in Europe. These indications, whicii flow naturally from the 

 theory, I have found also to be verified in the plains of western America, 

 where the physical circumstances are relatively the same. 



1. We are liable to estimate altitudes too low, when the observation 

 of the barometer have been in the morning or in the evening ; when the 

 lower barometer being on the plain, the upper is in a narrow deep valley ; 

 when tlie wind blows strongly from the south ; finally, when the weather 

 is about to be, or is already, stormy ; — a case which gives rise to great 

 errors. 



2. We are liable, on the contrary, to estimate altitudes too high, when 

 the observation is made between noon and two or three o'clock, p. m.. 

 especially in the summer, and under a hot sun ; when the upper barometer 

 benig on the summit of a mountain, the lower is in a confined and domi- 

 nated ravine; when there is a strong northerly wind, especially if the ob- 

 servation be made on a mountain with such a wind striking on the steepest 

 side. 



It agrees, then, with these valuable notices, that the best period of the day 

 for barometric observations of the altitude of mountains, is just about noon ; 

 and that observations made early in the morning, or late in the evening, 

 are the most subject to errors, which may be referred to the ascending or 

 descending currents, which, at such hours, are taking place in the atmo- 

 sphere, and to the varying degree of its dryness. 



To all such rules, furnished by science and experience, I have conformed, 

 as far as I have been capable. Everything was to do in relation to the 

 subject in the west, when I first went there. After all, I have only laid the 

 foundations of a great and important work, which others will add to and 

 enrich ; and I have reason to hope, therefore, that the numerous results of 

 this kind, which I present in the Table of Geographical Positions, will be 

 received with indulgence. 



All these altitudes, with the exception of what is south of the entr&nce 

 of the Ohio, have been referred to the ordinary low water in the Missis- 

 sippi at St. Louis. The absolute height of the barometer at this point 

 was not known ; and my addressing myself to the reverend fathers the 

 Jesuits at the head of the University of St. Louis, and engaging them 

 in making meteorological observations, was the first approach towards 

 obtaining it. They acquiesced cheerfully in my proposal, as they 

 do with everything tendmg to the advancement of learning. The late' 

 reverend Mr. Vansweevelt charged himself with the task, for which I 

 gave him a fine cistern barometer of Troughton, and received afterwards 

 nineteen months of observations, made five times a day, and followed as 

 regularly as his official duties would permit, through the years 1835 and 

 1836. 



When Mr. Vansweevelt was obliged to discontinue his observations, 1 

 had the good fortune to find a successor, not less zealous, in Dr. Engel- 

 mann, who was provided with a syphon barometer, graduated in French- 

 inches, and who followed these observations with a regularity that was 

 unlooked for from a person so occupied otherwise with professional en- 

 gagements. The years to which these observations refer are 1837, 1838.. 

 1839, and some montlis of 1840. 

 7 



