536 



Major J. Herschel. 



scale. "We should not be far wrong in estimating the power of such 

 an instrument, not in proportion to its size, but in an even higher ratio. 

 Even in scale-balances there is an advantage in size ; though there it 

 is the absolute weight put into the scale which is measured, whereas 

 in the gravimeter we have a balance whose delicacy is measured by 

 the relative minuteness of the weight added. The torsion gravimeter 

 relies on the perfect obedience of the bifilar suspension, due to the 

 absence of friction. Until it can be practically shown that this obedi- 

 ence is, in practice, not perfect, it is an abuse of the leading principle 

 to refrain from drawing upon the resource it offers. This is what 

 is done when high constructive art and skill are exerted to keep down 

 the weight and size of parts instead of the contrary. 



The minor weight is turned as one turns the hand of a clock with 

 the finger. I cannot imagine why this one-sided action is preferred — 

 for I suppose it is preferred — to the obvious two-fingered action, which 

 comes into play in so many common practices where one wishes to 

 avoid displacing the central axis of motion. The effect is to give a 

 wobbling swing to the whole hanging system, increasing the risk of 

 jar and strain, besides displacing the centre, and thereby altering the 

 normal direction of the pull of the lower weight as well as the verti- 

 cally of the mirrors. [It is true that magnets are provided to relieve 

 this, but I hardly suppose any one desiring to make accurate observa- 

 tions would allow them to remain as preferable to the two-fingered 

 stop.] I have already remarked on the so-called " finely-polished 

 agate points," which I have been obliged to replace by steel ones, 

 because being very thin and of glass they soon got broken off. I do 

 not recognise any objection to their being of metal. 



A tripod stand is furnished with the instrument, and I always used 

 it ; but it is very unsuitable, for the following reason. A portable 

 tripod almost necessarily requires a large splay ; this involves risk — 

 even in the hands of a surveyor habituated to three-legged stands — 

 and risk of a kind which, in my opinion, is fatal in the case of such 

 an instrument as this. This is why I notice the stand. I doubt if 

 anything like the necessary permanence of condition could be looked 

 for in a fine wire which, when supporting such a weight as 600 grs., 

 had to sustain a jolt, or such a shock as would be caused by a slight 

 kick to one of the legs of the stand. I have no proof of this. It is 

 one of the tests I intended to inflict on the instrument. In the absence 

 of anything but a strong doubt, I can do no better than set it down 

 here for future trial. 



The last point which I shall dwell upon is one which has already 

 been noticed, the effect of torsion of the double wires in deferring the 

 position of unstable equilibrium beyond that of maximum gravity 

 resistance. As already pointed out, the possibility of choosing 90° as 

 the place of rest turns on the alliance of this torsion with the force of 



