106 SEC. 3. MEASUREMENT, 



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. In this is enclosed an air bubble, whose position on a 

 scale indicates the depth of water below the instrument. 



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, by 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 

 vessel, whereas the bathometer gives the mean depth taken over a certain 

 area, depending for extent upon the depth itself. 



It is thought that the bathometer may render useful service to the mariner in 

 warning him of changes of depth long before reaching dangerous ground ; 

 and the position of a vessel, when no astronomical observations can be taken, 

 may be ascertained by means of the instrument, provided the contour lines of 

 equal depths of oceanic basins were accurately laid down. 



42 Ic. Graphical Bathometer , after von Jolly. 



University of Munich. 



42 Id. Gravimeter. An instrument for the measurement of 

 the variations of the earth's attractive force, invented by J. A. 

 Broun, F.R.S., and constructed from his drawings by Dr. C. F. 

 Miiller, of Stuttgart. J. Allan Broun, F.R.S. 



The instrument consists of a weight suspended by two gold wires ; a single 

 wire fixed to the top of the weight and passing through its centre carries a 

 cylindrical lever ; when the lever is turned through 360 at the normal (say 

 southern) station, the torsion of the single wire thus produced carries the 

 weight round through an angle of 90. The forces then in equilibrium are, 

 the torsion force of the single wire and the attraction of the earth on the 

 weight, Avhich, as the two wires are no longer vertical, has been slightly raised 

 and seeks to attain its lowest point. 



On proceeding from a southern to a more northerly station the earth's 

 attraction increases ; the amount of this increase may be measured in two 

 ways : 



1st. The lever will require to be turned through more than 360 in order to 

 carry the weight to the height due to turning it through 90. (Had the station 

 been more southerly the lever would be turned through less than 360.) The 

 difference of the angle from 360 measures the increase (or diminution) of 

 weight. 



2nd. By removing a small portion of the weight, equal to that due to the 

 increased attraction of the earth, the weight can be turned through exactly 



