S/K Wll.I.IA.M SIEMENS, F.R.S. 371 



atmosphere presses equally upon the surface of the mercury in the 

 upper cup of the bathometer and upon the diaphragm below, :in<l 

 variations in the height of the barometer, therefore, exercise, per 

 w, no influence upon the instrument ; but inasmuch as the 

 iiHTciiry column exercises a preponderating gravitating influence 

 only in the measure of its superior density to the atmosphere 

 which the mercury replaces in the tube, it follows that changes in 

 atmospheric density must exercise an influence upon the readings 

 of the instrument. The atmospheric density depends upon baro- 

 metric pressure, temperature, and admixture of aqueous vapour, 

 the amount of which can be easily ascertained by readings of the 

 dry- and wet-bulb thermometers and the barometer at the time of 

 takin" 1 the bathometrical observations. These corrections have 



O 



been made and applied to the observations taken on board the 

 steam-ship " Faraday " ; the readings, however, having been taken 

 at sea, the air was regarded as saturated with vapour, and the 

 tension of the vapour at the temperatures has been employed. In 

 ordinary usage of the instrument these corrections might be 

 neglected without serious error, or a table might be constructed 

 giving the amount of these corrections for observed changes of 

 the barometer and thermometer. 



GEOLOGICAL INFLUENCES. The readings of the bathometer 

 depend upon the inferior density of sea- water as compared with 

 the solid constituents composing the earth's crust, which have 

 been taken, in the calculation at page f>73, as 2763. No account 

 was taken, in assuming the above average density of the earth's 

 crust, of the presence of denser materials, such as metallic ores, 

 heavy spar, &c., on the one hand, or of subterranean cavities on 

 the other. But these abnormal occurrences are not frequent on 

 dry land, being chiefly confined to mountainous districts, and 

 may be assumed to be of less frequent occurrence in the great 

 depressions constituting the sea-basins. Their relative effect upon 

 total gravitation, as measured upon the surface of the water, is 

 less, moreover, than it would be if measured upon the solid 

 surface, on account of their greater distance from the instrument. 

 The uniform density of the sea is an element eminently favourable 

 to the attainment of uniform indications on its surface. 



GEOGRAPHICAL INFLUENCES. The configuration of the bottom 

 of the sea below the instrument must also exercise a sensible 



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