690 
DR. C. W. SIEMENS ON DETERMINING THE DEPTH OE THE SEA 
prising, however, a small bubble of air, which can be made to occupy a central position 
in the glass tube by raising or lowering the set-screws. 
If a weighty object is approached to either extremity of the connecting- tube an 
attractive influence will be exercised upon the mercury, tending to a rise of level in the 
reservoir near at band, at the expense of the more distant reservoir ; and this disturb- 
ance of level between the two reservoirs must exercise a corresponding effect upon the 
index of air in the horizontal glass tube, moving it away from the source of attraction. 
The amount of this movement must be proportionate to the attractive force thus 
exercised, and is considerable, because the transverse cross section of each reservoir- 
tube is 60x300=^18,000 square millims., whereas the section of the glass tube is only 
about 3 millims. ; the motion produced by the effect of gravity is thus increased 
3000-fold, and could easily be increased, say 30,000-fold, by simply increasing the hori- 
zontal area of the transverse or reservoir- tubes. Variations of temperature have no 
effect upon this instrument, because the liquids contained on either side of the index of 
air are precisely the same in amount ; and the total expansion of the liquids is compen- 
sated for by an open stand-tube rising up from the centre of the connecting-tube, 
through which the apparatus can be easily filled. By means of this instrument the 
effect of 1 cwt. approached to one end or the other of the mercury connecting-tube 
causes a sensible motion of the air index. 
It is suggested that an instrument of this description may be employed usefully for 
measuring and recording the attractive influences of the sun and moon which give rise 
to the tides. The instrument, which is of simple construction and not liable to derange- 
ment from any cause, would have to be placed upon a solid foundation with its 
connecting-tube pointing east and west, records being taken either by noting the position 
of the index upon the graduated scale below, or by means of a self-recording arrange- 
ment through photography. 
This mode of multiplying the effect produced by gravitation is applicable also to 
the bathometer ■ and one of these instruments was shown which was fitted with a 
spiral glass tube laid horizontally upon the upper surface of the bathometer upon a 
regularly divided scale, which horizontal tube is connected at one end with the upper- 
most chamber of the bathometer above the mercury, while the other end remains open 
to the atmosphere. The space above the mercury in the upper chamber is filled by 
preference with oil, which terminates in the horizontal spiral glass tube at a point 
which will vary with the total attractive influence of the earth, and thus furnish 
a means of reading the instrument. The electric contact arrangement described 
in the paper is thus rendered unnecessary, and the reading of the instrument much 
simplified. 
