Vill PREFACE. 
rical observations made in that part of the State should be carefully revised and recom- 
puted, with the aid of the table of corrections which we were then able to prepare from 
our three years’ series of observations, which had been specially made in order that. we 
mieht be furnished with the necessary data for that purpose. 
This part of the work was, therefore, that which first received our attention. The 
tables in question were computed and published under the title of “Contributions to 
Barometric Hypsometry, with Tables for Use in California.” Then came the laborious 
operation of recalculating and arranging the barometrical observations at the various sta- 
tions in the gravel region, the corrections indicated by the tables being applied to them. 
The method of applying the corrections in question, and the reasons for their use, have 
been fully explained in the “Barometric Hypsometry,” which was issued in 1874. A 
supplement, forming the fifth chapter of that work, was published in 1878, in which 
the practical value of our tables is demonstrated, by a bringing together for comparison 
and discussion of the results obtained by working over the many hundred observations 
which had been taken in the gravel region. In this division. of the work it is shown 
that the use of the tables in question has reduced the error of the results, on the average, 
by fifty per cent; and I may here add, after having made a careful examination of the 
whole subject, that there is no method of observation by which barometric results can be 
freed from the class of errors which these tables are intended to remove, neither is there 
any kind of corrections which can be substituted for those furnished by our work. 
All these laborious computations and investigations of the barometrical data were made 
by Professor Pettee, or by assistants employed under his direction. It must be noticed, 
however, that a portion of the altitudes given in the body of this work have not had 
the tabular correction applied to them. Consequently they differ slightly, in numerous 
instances, from those published in tabular form in Appendix C, which presents in one 
body the results of all the later and more trustworthy determinations made in the gravel 
region proper. 
During the time when the earlier part of our surveys in the hydraulic mining district 
were being made, we had the work of the three observers at the regular fixed stations, 
which we could use in calculating the results of our observations taken at various 
points in their respective vicinities. During the re-examination, last year, by Pro- 
fessor Pettee, however, of certain portions of the mining counties, as mentioned further 
on, he had not, to the same extent, the advantages of corresponding station barometers ; 
but by skilful combination of observations of the aneroid with the mercurial barom- 
eter, of each of which kinds he had two, the latter being used as station instruments 
for short periods at important central points, he was enabled to secure very trustworthy 
results, probably but little, if-at all, inferior in value to those obtained in former 
years.* 
* In the Table of Barometric Altitudes (Appendix C), those results to which an asterisk is prefixed are from 
observations made by Professor Pettee in 1879, as well as some others, as specified in the headings of the groups 
of localities. All the elevations given in the list of places north of the Middle Yuba are from the same source. 
