Abstract
THE memoir before us is concerned with the correction for refraction in geodetic operations between distant stations, especially those differing considerably in altitude. The author quotes Helmert's elaborate formula, which gives the correction as a function of gravity, atmospheric pressure, coefficient of expansion of air, tension of aqueous vapour, temperature, and vertical temperature gradient. The values deduced from the formula are compared with those obtained by observation over several bases in Italy and the Alps. The results are grouped both by months and by hours of the day; they show in a clear manner that there are both diurnal and annual variations in the refraction coefficient, which appear to be mainly due to the changes in the vertical temperature gradient. The following table shows the results of two series, the coefficients in the first column being deduced from the formula, and in the second by experiment. The third column gives the observed vertical temperature gradient.
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