Publisher's Synopsis
Excerpt from On the Use of the Barometer on Surveys and Reconnaissances: Being a Compendium, Without Plates, of No. 15 of the Professional Papers of the Corps of Engineers
I may remark, in the first place, that whenever the read ings of the barometer are referred to in the following pages, those of the barometer reduced to 320 Fahrenheit are meant. In the barometric formula of Laplace and others, a term has been introduced to take into account the effect of the ex pansion and contraction of the mercurial column by heat, in order to reduce the readings to what they would have been bad the temperature of the instrument been always at the freezing point. But it is equally accurate and much more convenient to reduce each reading in the first place to the freezing point by the tables which have been jpre pared for the purpose. By adopting this course, the column so reduced, when plotted, shows the movements of a natural atmosphere, and their peculiarities can be studied with advantage; whereas the readings of the barometer not so reduced give so irregular a curve, the movements being masked by the ever.varying temperature of the instrument, that it is scarcely possible to discover any law guiding them, if such a law exists.
I also wish to point out that, unless special mention is made to the contrary, the formula used in the computations is the one found in Professional Papers of the Corps of Engineers, No. 15, only omitting the special correction for the moisture in the atmosphere. It is a translation of the formula of Plantamour. This formula differs from the one prepared by Guyot for the Smithsonian Institution, which is, in fact, the formula of Laplace, by a very small change in the barometric constant. Plantamour adopts the num ber while Guyot gives This slight change causes the difference of altitude to be greater by the former formula than by the latter by a little less than four feet for each thousand feet of difference of altitude.
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