vni. FORCE. 85 



open stand tube rising up from the centre of the connecting tube through 

 which the apparatus can be easily tilled. 



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 arrangement through photography. 



4 



42 Ib. The Bathometer. An instrument for measuring the 

 depth of the sea without the use of the sounding line. 



Dr. Siemens. 



The total gravitation of the earth, as measured on its normal surface, is 

 composed of the separate attractions of all its parts, and the attractive 

 influence of each equal volume varies directly as its density, and inversely as 

 the square of its distance from the point of measurement. 



The density of sea water being about i-026, and that of the solid con- 

 stituents composing the crust of the earth about 2-763 (this being the mean 

 density of mountain limestone, granite, basalt, slate, and sandstone), it 

 follows that an intervening depth of sea water must exercise a sensible 

 influence upon total gravitation if measured on the surface of the sea. 



The bathometer consists essentially of a vertical column of mercury con- 

 tained in a steel tube having cup like extensions at both extremities, so as to 

 increase the terminal area of the mercury. The lower cup is closed by means 

 of a corrugated diaphragm of steel plate, and the weight of the column of 

 mercury is balanced in the centre of the diaphragm by the elastic force derived 

 from carefully tempered spiral steel springs of the same length as the column 

 of mercury. 



One of the peculiarities of this mechanical arrangement is, that it is para- 

 thermal, the diminishing elastic force of the springs with rise of tempera- 

 ture being compensated by a similar decrease of potential of the mercury 

 column, which decrease depends upon the proportions given to the areas of 

 the steel tube and its cup-like extensions. 



The instrument is suspended a short distance above its centre of gravity in 

 a universal joint, in order to cause it to retain its vertical position, notwith- 

 standing the motion of the vessel, and vertical oscillations of the mercury are 

 almost entirely prevented by a local contraction of the mercury column to a 

 very small orifice. The reading of the instrument is effected by means of a 

 glass tube on the top, which connects the upper surface of the mercury with a 

 liquid of less density, the terminal extremity of which on a scale represents 

 the depth. 



Variations of atmospheric pressure have no effect upon the reading of this 

 instrument ; but a correction has to be made for variations of atmospheric 

 density as affecting the relative weight of the mercury column, which correc- 

 tion might be avoided, however, in excluding the atmosphere. 



From both the upper and lower surface of the mercury, and connecting the 

 extremities of the column, the only necessary correction is that for the 

 effects of latitude, which may be calculated as depths in fathoms, and tabulated 

 for use with the instrument. 



The readings of the instrument have been checked by actual soundings 

 taken by means of Sir William Thomson's steel wire sounding- apparatus ; 

 and the comparable results agree in all cases as closely as could be expected, 

 considering that the sounding line gives the depth immediately below the 



