WILLIAM .s/A.I/AA.s, /-.A'..V. 365 



provided with a closed stopper, which is screwed down when the 

 instrument is not in use, and released for the access of atmospheric 

 pressure shortly before observations are about to be taken. The 

 1< >\\ IT portion is closed by means of a thin diaphragm of corrugated 

 pinto of steel, similar to the corrugated plates used in the con- 

 struction of aneroid barometers. The centre of the diaphragm 

 rests upon a crosshead, to which two carefully tempered steel 

 springs are attached, which pass upwards on opposite sides of the 

 mercury column, and are held at the upper extremities by adjust- 

 ing-screws in the sides of the upper cup. The neck of the vertical 

 pipe where it opens out into the upper cup is nearly closed by 

 means of a disk or stopper of steel, perforated by a hole of only 

 2 millim. diameter, the object being to reduce the pumping-action 

 on board ship to a minimum. Before screwing-in this stopper the 

 tube is filled with boiled mercury up to about the middle of the 

 upper cup. 



AvAii.Anu; FORCE. The mercury column represents the poten- 

 tial of force resulting from the area of the lower cup, multiplied 

 into the height of column and the density of mercury. 



The instrument of which the results have been chiefly recorded 

 in the Table given further on has cups of 90 millims. in diameter 

 and a height of mercury of GOO millims., representing an avail- 

 able force of 51'9 kilogrammes susceptible to variation in gravi- 

 tation ; whilst the instrument of which the drawing is given has 

 cups of 50 millims. diameter and a mercury column of 500 

 millims., representing an available force of 13'o5 kilogrammes. 

 These amounts are amply sufficient to overcome by their variations 

 any slight frictional resistance in the liquid column or in the 

 diaphragm. But this frictional resistance is really eliminated 

 from consideration by oscillations of the vessel, which cause 

 certain pumping-action (kept within narrow limits by the con- 

 tracted orifice), and bring the diaphragm into the true mean 

 position, notwithstanding slight frictional resistances. 



RAXGE OF SCALE. Under this head we have to consider what 

 will be the effect on the instrument by a given change in the 

 total attraction. Assuming a diminution of gravitation equal to 

 8a J 37!oo( representing about 10 fathoms of depth, this would 

 be equalized by a reduction in the height of column of arVtfao 

 millim. = '001G2 millim. The column of mercury in rising 



