318 Dr. C. W. Siemens on [Feb. 24, 



account, gravity would be found to diminish upon the sea-level in the 

 proportion of the depth to -§y|-R, or as E. nearly. 



The proportion would be strictly correct if the interior of the earth was 

 of the same density as surface-rock ; but the coefficient here arrived at 

 has to be diminished in the proportion of the density of surface-rock to 

 the mean density of the earth, or in the proportion of about ~ ; the 

 author considers it safer, however, not to rely entirely upon these mathe- 

 matical deductions in constructing his working scale, which he prefers to 

 base upon comparison with the sounding-line. 



The author next describes a first attempt made by himself in 1859 to 

 construct an instrument to indicate upon a scale such slight variations in 

 total gravitation as would result from the ordinary variations of depth 

 below a vessel, and the difficulties he then encountered. These difficulties 

 he has since succeeded in overcoming in adopting an instrument of another 

 construction, which he proceeds to describe in detail, reference being had 

 to the accompanying drawings. 



This instrument, which he proposes to call a Bathometer, consists essen- 

 tially of a vertical column of mercury contained 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 thin steel plate, and the weight of the column of mercury 

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

 from two 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 

 parathermal, the diminishing elastic force of the springs with rise of tem- 

 perature being compensated by a similar decrease of potential of the mer- 

 cury column, w T hich 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, 

 notwithstanding the motion of the vessel ; and vertical oscillations of the 

 mercury are almost entirely prevented by a local contraction of the mer- 

 cury column to a very small orifice. The reading of the instrument is 

 effected by means of electrical contact, which is established between the 

 end of a micrometer-screw and the centre of the elastic diaphragm. The 

 pitch of the screw and the divisions upon the rim are so proportioned that 

 each division represents the diminution of gravity due to one fathom of 

 depth. 



Variations in atmospheric pressure have no effect upon the reading of 

 this instrument ; but a correction has to be made for variations of atmo- 

 spheric density as affecting the relative weight of the mercury column, 

 which correction might be avoided, however, in excluding the atmosphere 

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



