1825.] Barometrical Measurement of Heights. 171 



Mr. Newman has remarked that when the cover of the cistern 

 of the Englefield barometer is sufficiently porous to admit the 

 air with freedom, the pressure of the screw forcing the mercury 

 to the summit of the tube cannot fail to expel some portion of 

 the fluid through the cover, thus disturbing the adjustment of 

 the zero ; and that on the other hand a cover made sufficiently 

 strong to prevent the escape of the mercury will so obstruct the 

 free admission of the air that a very considerable time must 

 elapse before the barometer exhibits the correct pressure of the 

 atmosphere. To remedy these evils, Mr. Newman constructs 

 the cistern of cast-iron having a cover of very porous wood^ 

 The instrument being sufficiently inclined, the mercury ascends 

 to the summit of the tube, and is retained therein by a screw 

 capped with leather, which, passing upwards through the bot- 

 tom of the cistern, presses against the orifice of the tube. Pro- 

 vided this novel method of rendering the instrument portable 

 shall be proved to preserve the column of mercury free from the 

 admixture of atmospheric air, there can be no doubt that the' 

 iron cistern will be found preferable to one of wood and leather^ 

 varying in tension, &c. with the hygrometric state of the atmo- 

 sphere. 



General Observations. li? 



Two barometers filled with mercury of the same specific grfc 

 vity, placed near each other with their cisterns on the same 

 level, will have their columns (corrected for capillarity, capacity, 

 and inequality of temperature) observed at the same height 

 under every variety of pressure : otherwise, the corrected 

 heights will be inversely as the specific gravities. Hence if two 

 barometers compared at the base of a mountain stand at 30 in. 

 and 30*2 in. respectively, w^e must increase the pressures exhi- 

 bited at the upper station by the instrument having the denser 

 mercury in the ratio of 30 to 30*2 ; in lieu of adding to it, as is 

 more generally the case, the discrepancy observed at the base. 

 It will be in vain to attempt the determination of the difference 

 of level of places distant from each other, by means of the sta- 

 tionary barometer, unless the mercury is known to be of the 

 same specific gravity in all the instruments. 



If we incline a barometer out of a vertical line the difference 

 of level of the cistern and summit of the column will remain 

 nnaltered, yet as the scale laterally attached to the tube is 

 equally incUned with it, we measure the height of the column 

 in excess in the ratio of radius to the secant of the angle of incli- 

 nation. Nevertheless if the barometers at the two stations 



require a cistern of an inconvenient depth, the red might be double or treble the area 

 of (a horizontal section of) the bore of the tube, but made to describe a vertical space, 

 inferior in die same ratio to the one passed over by the index ; easily effected by a pro- 

 per arrangement of the teeth of a wheel attached to the glass rod and the endless screw of 



the index. _,,.»... .^-^ ih-> 



