I^otes on Barometei' Construction. 53 



notwithstanding variation of temperature the chamber 

 remained air-free for months, during which the apparatus 

 remained under the writer's observation. 



"The board carrying the completed barometer can be 

 unscrewed from the base and suspended on a wall. 



" The above described instrument is well suited for use as 

 a portable barometer. It is first emptied of mercury, with 

 precautions ensuring that dry air only can enter in replace- 

 ment of the quicksilver; for this object chloride of calcium 

 tubes are attached at A and at C. The stopcock A is then 

 closed, and C is stopped with a small cork. During travel- 

 ling moisture cannot penetrate into the tube, thus dried 

 carefully once for all. The board unscrcAved from the 

 tripod, with its attached glass instrument, is fitted into a 

 padded case, which can then be carried suspended over the 

 shoulder as a fowling-piece ; with a sufficient^ strong case 

 even the brusque treatment incidental to railway carriage 

 can be safely borne. The mercury is carried with the 

 instrument in a securely corked stoneware bottle, of the 

 kind commonly used in commerce for the transport of small 

 quantities of this metal.* The third item of carriage is the 

 wooden triang-ular base. 



" Arrived at the observing station the tripod is screwed 

 on, the previously dried mercury (the warming of which is 

 now quite unnecessary, and which indeed was perhaps 

 superfluous on the first occasion) is poured in, and within a 

 quarter of an hour after the minutely described routine of 

 filling, the barometer is ready for observation. 



" This form of the barometer is recommended for isolate 

 barometrical stations, and for similar positions ; the drying- 

 out takes place in the laboratory, the glass pieces for which 

 operation, attached to the board, are carefully packed and 

 sent in the usual box. The filling takes place on the spot. 

 If an assistant unqualified by previous scientific technical 

 education be employed, it might prove advantageous to 

 enclose the barometer with a glass case. Incidental to the 

 inspection of the station would be the replacement of the 

 upper surface of the mercury in the open tube, the verifica- 

 tion of the instrument, &c. 



* Stoneware bottles containing mercury are rendered relatively safe from 

 accident by a cover of several layers of brown paper securely pasted on to 

 their outer surfaces.— G. F, 



