THERMOMETRICAL MEASUREMENT OF HEIGHTS. 437 



pressure. Owing to these various causes, an observation of the boiling point, differ- 

 ing by one tenth of a degree from the true temperature, ought to be still admitted as 

 a good one. Now, as the tables show, an error of oue tenth of a degree Centigrade 

 in the temperature of boiling water would cause an error of 2 millimetres in the 

 barometric pressure, or of from 70 to 80 feet in the final result, while with a good 

 barometer the error of pressure will hardly ever exceed one tenth of a millimetre, 

 making a difference of 3 feet in altitude. 



Notwithstanding these imperfections, the hypsometric thermometer, or thermo- 

 barometer, is of the greatest utility to travellers traversing distant or rough countries, 

 on account of its being more conveniently transported, and much less liable to 

 accidents than the mercurial barometer. The best form for it is that contrived and 

 described by Regnault in the Annales de Chimie et de Physique, Tom. XIV. p. 202. 

 It consists of an accurate thermometer with long degrees, subdivided into tenths, whose 

 bulb is placed, about 2 or 3 centimetres above the surface of the water, in the steam 

 arising from distilled water in a cylindrical vessel, the water being made to boil by a 

 spirit-lamp. The whole instrument when closed is about 6 inches long ; when drawn 

 out for observation, about 14 inches. 



Table XXIV. of barometric pressures corresponding to temperatures of boiling 

 water, has been calculated by Regnault from his Tables of Forces of Vapor, and 

 published in the Annales de Chimie et de Physique, Tom. XIV. p. 206. It gives, in 

 millimetres of mercury, the barometric pressures corresponding to every tenth of a 

 Centigrade degree ; for greater convenience, the values for every hundredth have 

 been added. 



The accuracy of this table has been tested by direct observation by Mr. Wisse, a 

 traveller competent in such matters, who noted down simultaneously the tempera- 

 tures of the boiling point of water and the height of the barometer, in various parts 

 of the Andes, up to the summit of the volcano of Pichincha, including in his obser- 

 vations barometrical pressures ranging from 752 to 430 millimetres of mercury. 

 The agreement between the barometric pressures given here by Regnault and those 

 found by Wisse are very satisfactory, the differences never exceeding a few tenths 

 of a millimetre. See Annales de Chimie et de Physique, Tom. XXVIII. p. 123. 



Table XXV. is the same table, revised by A. Moritz, who, in a communication to 

 the Academic des Sciences, in October, 1856, called the attention to some slight 

 errors of computation in Regnault's table, and gave the corrected numbers for every 

 whole degree from 40 to 102 Centigrade. Those numbers are given here from 

 upwards, as published in the Journal de Plnstitut ; the values for every tenth of 

 a degree, and their differences, have been computed to fit the table for practical use. 

 The comparison of the two tables will show that the corrections mostly amount to a 

 few hurjdredths, and never exceed one tenth of a millimetre. 



Table XXVI. is table XXV. reduced to English measures. 



D 97 



