y “ ae 2 ; 
250 Temperature of Mercury in a@ Siphon Barometer. = 
placed the figure of a man sculptured in jade,* whose hand re 
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 figure on the magnetic car was made use of for this purpose.” 
diate a —_ =< - 
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Arr. IIlL—A Method of determining the Temperature of the 
Mercury in a Sip Barometer, from the observed upper and 
lower readings ; and of testing the accuracy of the instrument ; 
by Farranp N. Bevepict, Prof. Math. and Civil Engineering, 
Univ. of Vermont. 
Ir 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 if 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 t 
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 pe aJong 
cylinder, of a diameter equal to that of the barometric tube, and 
similarly exposed to the surrounding influences of temperatute. 
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 I 
Spects, and, to a great extent, necessarily so. The brass mount 
ing is a hollow cylinder with two rectangular orifices neatly OP” 
spinner Me BEES < 
* A hard stone, of variegated hue. 
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