680 
DR. C. W. SIEMENS ON DETERMINING THE DEPTH OE THE SEA 
Compensation for Temperature-effects. — The means of such compensation is provided 
in the mercury column. If this column were to consist of a plain cylindrical vessel, 
not subject to change in diameter by temperature, it is evident that its pressure upon 
t he diaphragm would be the same whatever the temperature of the mercury might be ; 
for with increase of temperature the height of the column would increase, and the 
density of the mercury decrease in precisely the same degree : such a column might be 
called one of uniform potential, and would not afford the means of compensation here 
desired. If, on the other hand, the column were made to consist of two shallow cups 
at top and bottom, connected by a tube of such diameter that its area, compared with 
that of the cups, might be neglected in calculation, it is evident that the potential of 
such a column would vary with the temperature in the ratio of the dilatation of 
mercury ; in other words, the absolute height of the column would remain practically 
the same at all temperatures, whereas the density of the mercury would vary in the 
well-knotvn ratio of -00018153 per degree C. If a spring could be found whose ratio 
of variation was less than that required for the mercury, it is evident that between 
these extreme forms one might be found in which the two ratios of variation would be 
exactly alike. The ratio of variation of the steel springs depends upon their degree of 
hardness ; and in the case of the instrument here referred to it amounted to -000258, or 
was in excess of the compensating power furnished by the mercury. Complete compen- 
sation could therefore in this case not be obtained, although the remaining error is 
extremely small, and was rendered practically inappreciable by allowing the compara- 
tively inelastic diaphragm to take a portion of the mercurial pressure. 
The proportion, as resulting from calculation, would at any rate have to be modified 
in order to allow for the linear expansion of the steel composing the tube as affecting 
its capacity ; hut this expansion proceeding also in an arithmetical ratio will only affect 
to a small extent the precise relative diameter to be given to the tube, without in 
any way disturbing the ratios of arithmetical increase upon which the compensation 
of the instrument is based. An easy verification of this arrangement, which may be 
called a parathermal system of adjustment between gravitation and elastic force, is 
furnished in suspending the complete instrument in the hot-air chamber in which the 
experiments for variation of elasticity were made, when the variations of temperature 
gradually and artificially produced within the chamber should remain without effect 
upon its reading. 
On subjecting the first instrument constructed on this principle to this test, a variation 
was discovered amounting to -00000125 per degree C., which was not corrected, however, 
in trying the instrument on board the steam-ship ‘Faraday;’ and the results then 
obtained, and given below, have had to be adjusted to this extent for variation in tempe- 
rature. 
Influence of variation in Atmospheric Density. — The atmosphere presses equally upon 
the surface of the mercury in the upper cup of the bathometer and upon the diaphragm 
below, and variations in the height of the barometer, therefore, exercise, per se , no 
