On Gravimeters. 



525 



poles. The corresponding increase of gravity is in donble this pro- 

 portion, i.e., 450 on 86,400, or 1 on 192, nearly. Weights, as mea- 

 sured by a constant force such as the torsion of a wire is supposed to 

 be, are affected in like proportion but inversely. Now the weight of 

 the mass suspended by the double wires is 3,725 grs. ; and the change 

 of weight, if one may so express it, from pole to equator, will be yt—f , 

 or 19*4 grs. 



We have seen that the range of the scale in the collimator will measure 

 a deflection due to a change of weight of 2*76 grs., or thereabouts. It 

 follows that the extreme change of gravity which can be looked for 

 would cause a deflection about seven times as great as what the scale 

 will measure. 



This is provided for by the auxiliary weights, of which there are 

 five, weighing 4*4 grs. each, and by the contrivance of the glycerine 

 well, which will now be described. Before quitting this part of the 

 subject, however, I should point out that one division of the scale 



• Q99 



seems to correspond to a change of pendulum rate of — - x 225 = 107 



19 4 



second per diem. 



The buoyancy of the glycerine in the well depends on the volume of 

 liquid displaced by the plunger, and therefore on the diameter of the 

 latter, which can only be got at awkwardly, in one place. To ascer- 

 tain this diameter I cut a slit in a piece of zinc plate, rather wider at 

 the mouth than at the inner end, and used it as a gauge, marking the 

 place where the narrowing width fitted the plunger. A piece of 

 copper wire, "05 in diameter, was found to fit it at the same spot ; 

 perhaps *048 would be more correct. 



The specific gravity of glycerine is 1'26 ; hence a cubic inch weighs 

 349*4 grs. With these data we find that one inch rise of glycerine 

 will buoy 0*628 gr. The cathetometer scale being divided metrically, 

 this corresponds to 0*0247 gr. per millimetre. The micrometer reads 

 to hundredths of a millimetre. 



The riders weigh, as nearly as I can determine, 4*40 grs. each. 

 Hence one rider has the same effect as 178 millims. of rise ; but the 

 scale only runs up to 150 millims., so that additional riders will be 

 requisite. 



I found by trial that 100 millims. rise of the glycerine buoyed about 

 2 grs. It was not a careful observation, but agrees fairly with the 

 above calculation ; so that it seems pretty certain that the riders are 

 too heavy. 



The total weight being 3,725 grs., the variation of gravity from pole 

 to equator (being as 193 : 192 nearly) will require an augmentation of 

 19*4 grs. This corresponds to 225 seconds, therefore 1 second corre- 

 sponds to 0*0867 gr., or 3*5 millims. of glycerine, if the above is 

 correct. 



