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 arc 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 

 Dr. Hildreth's barometer noticed in the last number of the Journal, 

 may depend in part upon the specific gravity of the metal with which 

 it is 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 meora results given by an in- 

 strument that is rendered by their combined influence so very varia- 

 ble and uncertain in its indications as the common barometer, must 

 be of little value.* 



D rr * It has 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 pol- 

 ished on its interior surface, and furnished with a hollow screw having 

 a square nut for the purpose of forcing it down with a key and making 

 ^ 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 as in the figure. Mercury is then to be pour- 

 ed in at B until the compound tube is full. The solid screw is then to 

 be put into its place, the apparatus inverted with the bottom in a basin 

 of mercury, and the screw removed, when the mercury will flow out 

 until it stands at C, about thirty inches above B. The sere w may then be 

 returned to its place again and the mercury boiled in vacuo. When it has 

 become cool, a second immersion will cause the mercury to flow into the 

 B 5 glass tube which will thus be filled in vacuo with mercury thoroughly 



freed from air without any danger of fracture. As however, the vacuum above C, 

 would not be quite 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. 



VoLXX.— No. 2. 47 



