124 



Messrs. E. Threlfall and J. A. Pollock. 



has to be twisted or untwisted to bring the lever to its sighted 

 position. 



In carrying out the construction of the instrument on the principle 

 thus explained the following conditions have to be fulfilled : — 



The instrument must be portable, and must be able to withstand 

 the rough usage inseparable from travelling, without being put out of 

 adjustment. It must have a sensitiveness of at least one part in 

 100,000 of the value of " g," i.e., a change in the value of " g " 

 amounting to one part in 100,000 must be shown by the balance. 



These conditions have been satisfied in the following manner : — The 

 thread supports form part of a girder mechanism which is itself con- 

 tained in a thermally insulated tube. During transport the lever is 

 arrested by a mechanism which clamps it with a definite pressure in a 

 definite position. The end of the lever is observed by a microscope 

 which is always brought into the same relative position with respect to 

 the horizontal plane through the thread by means of sensitive striding 

 levels. It is shown as a consequence of the mechanical conditions 

 that the lever will be in unstable equilibrium when its centre of 

 gravity rises above the horizontal plane through the thread by about 

 3°. Consequently the accuracy with which the lever can be brought 

 to its sighted position is very great, for the position selected as the 

 sighted position is within a small fraction of a degree of the position of 

 instability. 



As it is necessary either to keep the balance in an atmosphere of con- 

 stant density or to correct the observations for changes in the barome- 

 trical pressure, the former course was decided upon, and consequently 

 the instrument is contained in an air-tight space. This involves working 

 the vernier axle through a stuffing box which must be practically Mo- 

 tionless, a condition satisfied by a sort of mercury sealing. 



The difficulty in making the apparatus arises from the fact that 

 quartz fibres, though infinitely better than any other material, are not 

 really sufficiently perfect in their elastic properties for the present pur- 

 pose, and it is only by a judicious balancing of advantages that it is 

 possible to arrive at the necessary sensitiveness. Even after two years' 

 twisting the thread of the instrument still undergoes a continual, 

 though small, viscous deformation; this, however, becomes sensibly 

 constant, and can be allowed for. 



A further complication arises from the fact that as the temperature 

 rises the quartz becomes stiffer, so that at a given station the circle 

 readings are a function of the temperature. We have found that the 

 relation between the circle reading and the platinum temperature is a 

 linear one at ordinary temperatures. 



An essential feature of the apparatus, therefore, is a platinum wire 

 thermometer placed alongside the thread. 



The following statement shows concisely the effect on a determi- 



