68 



Description of a thermometrical Barometer fur measuring Altitudes. 

 By the Rev. Francis John Hyde Wollaston, B.D. F.R.S. Read 

 March 6, 1817. [Phil. Trans. 1817, .p. 183.] 



The author's attention having been drawn to the variations in the 

 heat of boiling water, as corresponding with changes in atmospherical 

 pressure and in the height of the barometer, he was led to construct 

 the instrument described in the present paper, for the purpose of 

 measuring heights with greater accuracy and convenience than by the 

 common barometer, or by the methods formerly devised by Fahren- 

 heit and Cavallo. 



The author constructed his thermometer with different scales, from 

 an uncertainty how far their sensibility might be carried; and in one 

 instance the bore of the tube was so minute, and the ball so large, 

 that every degree on Fahrenheit's scale was equal to ten inches. The 

 instrument, however, with which the greatest number of observations 

 were made, had a scale of 3'98 inches to every degree, and each de- 

 gree is divided into 100 parts upon the scale, and into 1000 by a 

 vernier. On comparing this thermometer with a good barometer, 

 it was found that the two instruments agreed equally well in all parts 

 between 30' 68 inches and 28' 23 inches. The result was, that a dif- 

 ference of 1 of Fahrenheit's scale is occasioned by 0'589 inch on the 

 barometer; 30'603 inches (corrected) on the barometer, =213'367 

 on the thermometer, and28'191 inches barometer, = 209'263 ther- 

 mometer. 



Having thus ascertained the delicacy and capability of the instru- 

 ment, the author proceeds to describe the best mode of constructing 

 it, which is further illustrated by an annexed drawing. To the ver- 

 nier is attached a small lens of an inch focus, which, on account of 

 the smallness of the bore of the tube, is useful in observing the height 

 of the mercurial thread, and by having no lateral motion, confines 

 the view to the same direction, thus preventing parallax. 



The boiler of this instrument is a tin cylinder 5 '5 inches deep, and 

 1*2 inches in diameter, with an external cylinder 1*4 inch diameter, 

 for preventing the transmission of heat. The bottom is single, and 

 the bulb does not dip into the water, but is exposed to the steam only; 

 and a bell-tent protects the lamp and boiler from the wind. The lamp 

 is filled with oil, to which a sufficient quantity of tallow is added to 

 make it congeal at common temperatures. 



Having tried threads of various* thicknesses, the author advises the 

 scale of an inch to a degree as best adapted to all ordinary purposes ; 

 for when finer than this, it is almost impossible to give such strength 

 to the bulb as to force the column of mercury accurately to the same 

 height on repetition of the boiling, by reason of the resistance from 

 friction in the tube. 



With an inch scale, the variations of the barometrical thermometer 

 are to those of the common barometer as 5 : 3 ; and the sensibility of 

 the instrument is such, that the difference of temperature required to 

 make water boil arising from the height of a common table, is imme- 



