PREFACE 



TO THE THIRD EDITION 



A NEW series of Hygrometrical Tables, based on Regnault's Table of Elastic 

 Forces of Vapor, has been published by Mr. Glaisher, in London, 1856. As, 

 however, the Psychrometrical Table has not been computed from Regnault's 

 formula, but by means of empirical factors, the results differ from those 

 contained in Table VII. B. A table containing Glaisher' s empirical fac- 

 tors, therefore, has been added, and will be found on page 144 B. 



Table XVIII. of the Barometrical set, C, page 72, of the Second Edition, 

 for reducing to the freezing point the Barometers with glass or wooden 

 scales, copied from the Instructions of the Royal Society of London, and 

 which is reprinted in most of the English works on Meteorology, having 

 been found erroneous, a new table has been computed and substituted for 

 it. As a large number of observers still use barometers with wooden scales, 

 it was found advisable to enable them to make the needed interpolations at 

 sight, by giving the corrections for every degree of the thermometer, from 

 to 100 Fahr., and for barometric heights ranging between 26 and 31 inches. 



The small Table VI. D, page 48, of the Hypsometrical Tables by the writer, 

 having been found useful for rapid computation of approximate results, a 

 larger one of the same description, which allows to make at sight every 

 interpolation, has been added, on page 92, as Table XIX'. The scientific 

 traveller, wishing to determine, when ascending a mountain, the elevation 

 of the physical or geological phenomena that he meets with, such as the 

 stations of remarkable plants, limits of zones of vegetation, the geologist 

 who uses the aneroid barometer for geological sections, the engineer who 

 wishes to know, on the ground, approximately, his results, will find it con- 

 venient to obtain the relative heights indicated by their instrument by a 

 simple multiplication. The use of the table is explained page D 90. 



Some of the decimals in the smaller Table VI. D, page 48, above men- 

 tioned, have been slightly altered in order to make both tables agree. 



In set E of Meteorological Corrections, a table of corrections derived by 

 Professor C. Dewey from the hourly observations of Professor Snell, at 

 Amherst College, has been added, which will be of service especially to 

 the numerous observers in New England and in the neighboring States. 



The errata indicated in the Second Edition, and a few unimportant ones 

 found since, have been corrected. No other changes have been made in 



this edition. 



A. GUYOT. 



PRINCETON, N. J., April, 1859. 



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