130 LECTURES TO SCIENCE TEACHERS. 



that the elastic force of the spring diminishes with an 

 increase of temperature in an arithmetical proportion, and 

 in a proportion which is inferior to the dilatation of mercury. 

 Therefore by diminishing the amount of mercury between 

 the diaphragm and the surface I arrive at a point where 

 the total variation in the potential force of this column of 

 mercury varies by temperature in the same proportion as 

 the spring varies also ; and if these proportions are properly 

 adjusted the instrument before you will be parathermal or 

 remain uninfluenced by variations of temperature. That 

 was a very important point to establish, because if the 

 instrument had to be maintained at a uniform temperature, 

 or if calculations had to be made to adjust it for tempera- 

 ture, those variations would very likely overbalance and 

 swamp the indications of the instrument due to depth, and 

 so would have rendered it useless. It was chiefly owing to 

 this difficulty that I did not succeed sufficiently well in my 

 first attempt. 



The atmospheric pressure which determines the height 

 of the mercury in a barometer exercises no influence 

 on this instrument, because it is exerted upon the Iqwer 

 side of this diaphragm as well as upon the upper surface 

 of the mercury, and assuming the pressure to be the 

 same at each level or proportionately the same, there 

 will be no influence ; but there is a slight influence due to 

 variations in the atmospheric density as distinct from 

 atmospheric pressure. The weight of this mass of mercury 

 in the tube is only in as much as the mercury is heavier than 

 atmospheric air. If the difference of density between 

 mercury and air weVe small, then any increase in atmos- 

 pheric density would not only influence the pressure of the 

 mercury on this diaphragm very materially, but enormous 

 as the difference is between the density of mercury and air, 

 yet a variation in atmospheric density exercises an influence 

 upon the potential of this column of mercury, and this 

 variation will be sufficient to influence the delicate indica- 

 tions which we expect from it. This has been eliminated 

 by suspending the instrument in an air-tight casing. It is 

 now raised out of its casing for you to see it, but when in 

 use it descends into the case and there swings freely on its 

 gimbals, and although there is the atmospheric pressure 

 inside the casing of average density, yet, as this casing is 



