RN 3 
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On Storms and Meteorological Observations. 365 
Dr. Young states that mercury once distilled has a specific gravity 
varying from 13.55 to 13.57—that the mercury employed in filling 
barometers has commonly ! a specific gravity of 13.6, and that Boer- 
haave by 511 distillations once obtained it as high as 14.11. What 
would be the condition of the ‘science of astronomy if an able phi- 
losopher were to find occasion for the remark, that the are employed 
by astronomers in measuring the angular distances of the heavenly 
bodies and divided by them into ninety degrees was commonly a 
quadrant or quarter of a circle? The low. state of the mercury in 
a Dr. Hildreth’s barometer noticed in the last number of the Journal, 
‘hig may depend in part upon the specific gravity of the metal with which 
itis filled. Whether these discrepancies depend upon the presence 
_ Of a foreign substance or supposing the mercury to be pure; by what 
_ methods a given specific gravity may be obtained seems not to have 
_ been made the subject of accurate investigation. ‘The other sources 
of error depending on the presence of air in the tube; the unequal 
pressure of the bag containing the mercury, etc. are not particularly 
noticed here, but it is evident that the mean results given by an in- 
ble and uncertain in its indications as the common barometer, must 
be of litle value.* a : | af 
D ssoe Athan occurred to me that when a considerable manufacture of ba~ 
rometers is carried on, the arrangement in the margin might be employ- 
ed with advantage for filling them in vacuo, boiling the mercury and 
thus freeing them completely of air. AB is a gun barrel carefully 
ished on its interior surface, and furnished with a hollow screw ha 
AG an air-tight joint at A, and with a smaller solid screw at B. The barom~— 
eter tube, thoroughly dried, is to be cemented into the hollow screw and 
connected with the barrel asin the figure. Mercury is then to be pour- 
ed in at B until the compound tube is full. e solid screw is then 
be put into its place, the apparatus inverted with the bottom ina b 
of mercury, and the screw removed, when the mercury will flow 
until it stands at C, about thirty inches ab The screw may then 
a 
M become cool,.a second immersion will cause the mercury to flow into the 
BS glass tube which will thus be filled in vacuo with mereury thoroughly 
freed from air without any danger of fracture. As however, the vacuum above C 
Would not be quife perfect in the first instance, the operation might be repeated. 
The tube being filled again at B and inverted, all the metal containing air would 
flow out first, and the success of the process be rendered complete and certain. To 
prevent the oxidation of the iron and dust from getting in, the barrel may be kept 
constantly filled with mercury. 
Vol. AX to. 2 47 
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strument that is rendered by their combined influence so very varia~ 
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; ai tee at : d th boiledi When it has 
