vni. FORCE. 105 



This instrument consists of two horizontal tubes of wrought iron, ter- 

 minating at each end in a horizontal tube of cast iron. The first-named 

 horizontal tubes are partiall}* closed at their extremities, and communicate 

 with the transverse tubes below their horizontal mid-section. The transverse 

 tubes communicate 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 levelling screws is filled 

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

 mercury also fills the whole of the longitudinal connecting tubes ; the upper 

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

 filled with alcohol, comprising, however, a small bubble of air, which can be 

 made to occupy a central position in the glass tube b} r raising or lowering 

 the levelling 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 horizontal 

 glass tube, moving it away from the source of attraction. The amount of 

 this movement must be proportional to the attractive force thus exercised. 

 Variations of temperature have no effect upon this instrument, because the 

 liquids contained on either side of the bubble of air are precisely the same 

 in amount ; and the total expansion of the liquids is compensated for by 

 open stand tubes rising up from the centre of the connecting tubes 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 \vest, records being taken either by 

 noting the position of the index upon the graduated scale below, or by means 

 of a self-recording photographic arrangement. 



42 Ib. 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 pressure of the mercury 



