320 On determining the Depth of the Sea. [Feb. 24, 



ubes below their horizontal mid section. The transverse tubes commu- 

 nicate also by means of a horizontal glass tube of 2 millims. diameter at 

 a superior level to the former. 



The whole apparatus being mounted upon three set screws is filled to 

 the level of the half-diameter of the transverse tubes with mercury, which 

 mercury fills also the whole of the longitudinal connecting-tube ; the upper 

 halves of the cast-iron transverse tubes and the glass connecting-tube are 

 filled with alcohol tinted with cochineal, comprising, 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 hand, at the expense of the more 

 distant reservoir ; and this disturbance of level between the two reservoirs 

 must exercise a corresponding effect upon the index of air in the hori- 

 zontal 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 60 x 300 = 18,000 square millims., whereas the sec- 

 tion 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 in- 

 creased, say 30,000-fold, by simply increasing the horizontal 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 compensated 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 mo- 

 tion 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 derangement from any cause, would 

 have to be placed upon a solid foundation with its connecting-tube point- 

 ing 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 

 arrangement through photography. 



This mode of multiplying the effect produced by gravitation is shown 

 as applicable also to the bathometer ; and the author showed one of these 

 instruments 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 uppermost 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 



