26 Plan for a Self- Registering Barometer. [Jan. 



TV,— Description of a Plan for a Self-Registering Barometer, and 

 for the Construction of Metallic-tube Barometers. — By William 

 Gilchrist, Esq. of the Madras Medical Establishment. 



A Barometer that will record constantly, and with scientific accuracy, 

 the varying pressure of the atmosphere, is, so far as I know, a deside- 

 ratum in the study of Meteorology. This important object may, I con- 

 ceive, be accomplished by suspending a barometer tube from one end of 

 a balance — an apparatus being connected with the other to record the 

 oscillations occasioned by the varying weight of mercury in the tube^ 

 or, more correctly, the varying pressure of the atmosphere on the top of 

 the trbe, and of which the height of the mercury within is an exact mea- 

 sure. 



In principle the barometer is a balance of a very delicate kind, its 

 function being to weigh a given column of the atmosphere, by counter- 

 poising its weight with a column of mercury — like the balance, 

 therefore, this instrument requires simplicity of construction, since 

 a complicated structure would impede its free working, and, of 

 consequence, falsify its indications. The nearer the principle of the ba- 

 lance, therefore, is approached in a barometer intended to register its 

 own movements, the better is it adapted for the exact purposes of science. 

 In the plan about to be described, this object will be found fully attained, 

 since a balance is made the medium of register. 



The accompanying is a sketch of the instrument (PI. 15. fig. 1.) — A is 

 a tube, suspended from one end of the balance B. The tube is of cast iron, 

 cast in the shape required, thereby avoiding the tediousness of boring. 

 The upper part, it will be observed, is considerably larger than the lower. 

 I purpose making the interior of this part with a uniform area of two 

 square inches. The evident object of this is to gain power, for the larger 

 the area of the bore, the greater is the weight of mercury displaced or 

 added, according to the depression or elevation of the column of mercury, 

 and consequently the apparatus has a proportionally greater power to 

 overcome resistance, which at most can be very small. A cubic inch 

 of distilled mercury weighs 3.434 grains, which divide by 1000 = 3.434 9 

 so that an atmospherical alteration that would affect the barometer 

 •TffVo of an inch gives nearly 3| grains of power — two square inches of 

 area, then, will give nearly a power of 7 grains to indicate the third 

 decimal figure, and nearly 70 grains to indicate the second or T <y o of an 

 inch ; so that it is evident the above power is amply sufficient to over- 

 come all obstacle, and to admit of recording variations of the mercurial 

 column to t^oo of an inch, a degree of minuteness amply sufficient to 

 meet the most exact demands of science. The upper part of this tube 

 must be bored accurately—as on the uniformity of caliber of this portion 

 of the tube the correctness of the instrument essentially depends. This 



