680 BEMAEKS ON THE BAliOMETER. 



whatever causes the warning, whether wind or a fall of rain or snow, 

 will continue. 



Sometimes severe weather from an Equatorial direction, not lasting 

 long, may cause no great fall of the Barometer, because followed by a 

 duration of wind from Polar regions ; and at times it may fall considerably 

 with Polar winds and fine weather, apparently against these rules, because 

 a continuance of Equatorial wind is about to follow. By such changes as 

 these one may be misled, and calamity may be the consequence if not 

 thus forewarned. 



This ends Admiral FitzRoy's remarks : what follows is taken chiefly 

 from " Instructions in the use of Meteorological Instruments " issued by 

 the Meteorological Office. 



Further Remarks. — The Barometer was invented in 1643 by Torricelli, 

 consequent on attempts to raise water by pumps from a deep well in 

 Florence ; by no method could the water be made to rise higher in the pump 

 tube than about 32 feet. Experimenting on this, he found mercury in a 

 short tube more manageable for showing changes in atmospheric pressure. 



Various fluids have been used for filling the tube, but those giving off 

 vapour at low temperatures are very unsuitable, the space above the fluid 

 being far from a vacuum. Mr. J. B. Jordan invented a glycerine 

 Barometer, having a length of about 32 ft., showing slight variations in 

 pressure very clearly. For scientific purposes, however, mercury is the 

 only suitable fluid for Barometer tubes. 



The Barometer now recommended by the Meteorological Office for use 

 at sea, is known as the Kew Marine Barometer. Each of these instru- 

 ments is tested at the Kew Observatory, and the scales correctly marked, 

 the errors due to the capacity of the cisterns, varying size of the tube, &c., 

 having been allowed for. By means of the Veryiier, the height of the 

 mercurial column may be read to one-thousandth part of an inch. For 

 the prevention of pumping, or oscillation of the column of mercury, the 

 lower part of the tube has a contracted bore down to the cistern, and the 

 upper part as far as the scale portion is, of even a finer bore. In the 

 larger bore, between these two fine bores, there is inserted a pipette, or 

 funnel, spout downwards, the object of this being to intercept any air 

 which, creeping up the sides of the tube, would, by getting into the 

 vacuum at the top of the tube, completely upset the working of the 

 instrument. The best test for moisture or for particles of air, between 

 the column of mercury and the side of the glass tube, is the magnifying 

 glass, by means of which the existence of such matter is easily detected. 



Sometimes, though very rarely, a particle of dirt, or a bubble of air, 

 lodges in the very fine contraction of the tube of a marine Barometer, and 

 completely stops the action of the instrument. Whenever, therefore, a 

 marine Barometer becomes stationary or inactive, when it evidently ought 

 to be moving under the influence of atmospheric changes, there being no 

 «yidence of fracture of the glass, the cause may be surmised to be of this 

 nature. It should then be taken down, the mercury allowed to fill the 

 tube, and the instrument put aside, in an inverted position, for a few hourau 

 On replacing it, the cause of the stoppage will generally be found to have 

 been removed to a part of the tube where it oaa do no harm. 



