132 BULLETIN OF THE 



the changes which affect the column of air to be measured ; 

 namely, that column which rests upon the base station and ex- 

 tends upward to the level of the new station. To obviate this 

 difficulty, at least two devices have been resorted to. First, by 

 establishing bases at several different altitudes and referring each 

 new station to that base which lies most nearly in its own level, 

 the error, which is proportional to the desired difference of alti- 

 tude, has been reduced in amount. Second, by using mean 

 temperatures and mean humidities, in place of those recorded at 

 the hours of observation, and thus ignoring the small but actual 

 change of temperature that the column each day undergoes, it 

 has been found that better results have been obtained than by 

 the direct use ofthe coincident observations for density. 



In the method which I here propose, no observations are made 

 of the two factors of temperature and moisture; but their result- 

 ant, the density, or more strictly a co-efficient of density, is deter- 

 mined directly. Within or near the district which contains the 

 new station, I propose to establish two base stations, which 

 shall be separated as little as possible horizontally, and as much 

 as possible vertically ; I will then njeasure simultaneously the 

 pressure at each of the bases and at a new station ; and then, 

 assuming that the co-efficieut of density is unity, will estimate by 

 the aid of the usual barometric tables the diffei'ence of altitude 

 between the two bases, and the difference of altitude between the 

 new station and one of the bases. Then, by dividing the esti- 

 mated difference of altitude of the two bases by their actual 

 difference of altitude, previously and independently determined, 

 I will obtain the co-efficient of density, which, being applied to 

 the estimated difference of altitude between the new station and 

 one base, will give the required difference of altitude. 



This method is especially applicable to a mountain region, in 

 which the error of density is usually more important than the 

 error of gradients ; and it can be most economically employed 

 where the number of new stations referable to a single pair of 

 bases is large. 



Mr. Thomas Antisell read a paper, by Prof. C. E. Munroe, 



of the U. S. Naval Academy, on 



THE ESTIMATION OP MANGANESE AS PYROPHOSPHATE. 



(TAjs paper was offered in January, and is printed in the American 

 Chemist of February, 1877.) 



Mr. Antisell also made a communication, entitled by him 

 chemical remarks ON terrestrial geogony. 



