PHYSICS OF THE GLOBE. 199 



maim introduces an essay on the Best Time for Determining 

 Altitudes by the Barometer. In continuation of the labors 

 of Ruhlmann, Bauernfeind, Plantamour, and others, he shows 

 that the correct altitude can be deduced from observations on 

 summer afternoons, and that, too, without using the tempera- 

 ture of the upper stations, and even when the stations are at 

 considerable horizontal distances. His method depends on 

 the assumption that the equilibrium of the ascending cur- 

 rents is maintained and expressed by the known laws of ther- 

 mo-dynamics (as developed quite independently by Thom- 

 son, Reye, Hann, and others), and assuming that the ascend- 

 ing currents neither give any heat to surrounding air nor 

 receive any from it or from the sun, or other source. His 

 formula thus deduced, and employing for all his physical 

 constants the numbers ordinarily accepted as resulting from 

 laboratory experiments, enable him to compute altitudes up 

 to 7000 feet, with an extreme error of 15 feet when he uses 

 monthly means of observations in July at 1, 2, or 3 P.M. 



Lieutenant -Colonel R. S. Williamson, U.S.E., has pub- 

 lished a compendium of his paper on the Use of the Barom- 

 eter on Surveys, which is followed by a comparison of his 

 method with that of Professor J. D. Whitney, as described 

 in his work entitled "Contributions to Barometric Hypsom- 

 etrv." Colonel Williamson claims to have shown conclusive- 

 ly that Whitney's method gives over 40 per cent, more of 

 maximum and mean errors than does his own. 



Major Powell states, in reference to the hypsometric work 

 of his survey in Southern Utah in 1877, that it rests on a 

 primary base established at Mount Pleasant, at which bar- 

 ometric observations were made four times daily, and were 

 also made hourly for eight days of each month. All his 

 camps and observing-stations were connected with the base 

 by barometric observations; but it is recommended that a 

 special series of hourly observations be conducted for a few 

 years upon some of the Rocky Mountain peaks for the pur- 

 pose of correcting the barometric formula? now in use. For 

 topographic details much use has been made for some years 

 past of the orograph an instrument devised by Professor 

 A. H. Thompson, and which seems to have been lately rein- 

 vented in France. 



