DIFFERENTIAL GEAVITY METER COMMITTEE. 143 



position has been chosen for the weight relatively to a mark rigidly 

 connected to the fixed end of the spring, fulfilling the condition that in 

 this position the eqnilibrinm is stable at all the temperatares for which it 

 has hitherto been tested, while the unstable position of equilibrium is only 

 a few millimetres above it for the highest temperature for which the 

 instrument has been tested, which is about 16° C. 



The fixed end is rigidly attached to one end of a brass tube about 

 8 centimetres diameter, surrounding the spring and weight, and closed 

 by a glass plate at the upper end of the incline, through which the weight 

 is viewed. The tube is fixed to the hypothenuse of a right-angled triangle 

 of sheet brass, of which one leg inclined to it at an angle of about one- 

 fifth radius is approximately horizontal, and is supported by a transverse 

 trunnion resting on fixed V's under the lower end of the tube and a micro- 

 meter screw under the short approximately vertical leg of the triangle. 



The observation consists in finding the number of turns and parts of 

 a turn of the micrometer screw required to bring the instrument fronx 

 the position at which the bubble of the spirit-level is between its proper 

 marks to the position which equilibrates the spring-borne weight with a 

 mark upon it exactly in line with a chosen divisional line on a little scale 

 of 20 half- millimetres, fixed in the tube in the vertical plane perpendicular 

 to its length. 



The instrument is, as is to be expected, exceedingly sensitive to 

 changes of temperature. An elevation of temperature of 1° C. diminishes 

 the Young's modulus of the german silver so much that about a turn 

 and a half of the micrometer screw (lowering the upper end of the tube 

 at the rate of f millimetre per turn) produced the requisite change of 

 adjustment for the balanced position of the movable weight. About 

 1^ turn of the screw corresponds to a difference of ^-^^ in the force of 

 gravity, and the sensibility of the instrument is amply valid for ■:^^ of 

 this amount, that is to say, for ^ ,, q^q „ q difference in the force of gravity. 

 Hence it is not want of sensibility in the instrument that can prevent it 

 measuring differences of gravity to ^ ^ ^^q ^ „ ; but to obtain this degree of 

 minuteness it will be necessary to know the temperature of the spring to 

 within ^° C. I do not see that there can be any very great difficulty in 

 achieving the thermal adjustment by the aid of a water-jacket and a 

 delicate thermometer. To facilitate the requisite thermal adjustment I 

 propose, in a new instrument of which I shall immediately commence the 

 construction, to substitute for the brass tube a long double girder of 

 copper (because of the high thermal conductivity of copper), by which 

 sufficient uniformity of temperature along the spring, throughout the 

 mainly effective portion of its length, and up to near the sighted end, 

 shall be secured. The water-jacket will secure a slight enough variation 

 of temperature to allow the absolute temperature to be indicated by the 

 thermometer with, I believe, the required accuracy. 



M. MascarVs Instrument. 



In ' Comptes Rendus,' xcv. (2, 1882), p. 126, is an account of a 

 differential gravity meter by M. Mascart. 



He employs a siphon barometer with the shorter tube closed, and 

 containing gas to support the mercury in the longer tube. The gas 

 chosen is carbon dioxide, to prevent oxidation of the mercury. The column 

 of mercury supported is 1 metre in length. A scale is attached to the 



