250 Temperahire of Mercury in a Siphon Barometer. 



placed the figure of a man sculptured mjade* whose hand always 

 points to the south. This figure had motion in the hole, and turned. 

 In the years Yan Yeou, (from 1314 to 1320,) it was an object to 

 determine the position of the monastery of Yao mou ngan, and 

 the fi.gure on the magnetic car was made use of for this purpose." 



Art. III. — A Method of determining the Temperature of the 

 Mercury in a Siphon Barometer^ from the observed upper and 

 lower readings ; and of testing the accuracy of the instrument ; 

 by Farrand N. Benedict, Prof Math, and Civil Engineering, 

 Univ. of Vermont. 



It has long been known that a true determination of the tem- 

 perature of the mercurial column, is essential to the accuracy of 

 barometrical results. The apparatus now in use for this purpose 

 is a thermometer encased in the brass mounting,, with its bulb 

 contiguous to the tube of the barometer. While there can be no 

 doubt that the attached thermometer has answered a useful pur- 

 pose for indicating approximately the temperature of the mercu- 

 rial column, it is equally evident that its indications are not to be 

 relied upon in many cases within the requisite limits of exact- 

 ness. These cases, in the present state of science, are the most 

 common and generally the most interesting. When the subjects 

 of investigation are such as to admit of a choice of the places of 

 observation, as the vaults of observatories or large and deep cel- 

 lars, the temperature may be assigned with all necessary precis- 

 ion. But in the most of physical questions, like those relating to 

 the barometrical measurement of altitudes or to detecting the ho- 

 rary and diurnal variations of atmospheric pressure, the errors in- 

 cident to this mode of appreciating the temperature, under ordi- 

 nary circumstances, are necessarily considerable. To entitle the 

 attached thermometer to confidence, its bulb should be a long 

 cylinder, of a diameter equal to that of the barometric tube, and 

 similarly exposed to the surrounding influences of temperature. 

 But these conditions have proved difficult to satisfy. Bunten's 

 mountain barometer, which is probably the most perfect portable 

 instrument of the kind now in use, is faulty in each of these re- 

 spects, and, to a great extent, necessarily so. The brass mount- 

 ing is a hollow cylinder with two rectangular orifices nearly op- 



* A hard stone, of variegated hue. 



