124 REPORT — 1881. 



advantage of giving an automatic record of its behaviour. On the othet 

 hand the lever must introduce a very unfavourable element in the friction 

 between solids. 



M. d'Abbadie's method of obsei-vation by means of the pool of mercury 

 seems on the whole to be the best which has been employed hitherto. 

 But it has faults which leave ample fields for the use of other instruments. 

 The construction of a well of the requisite depth must necessarily be 

 very expensive, and when the structure is made of a sufficient size to 

 give the required degi'ee of accuracy, it is difficult to ensure the relative 

 immobility of the cross wires and the bottom of the well. 



Levels are exceedingly good from the point of view of cheapness and 

 transportability, but the observations must always be open to some 

 doubt on account of the possibility of the sticking of the bubble from 

 the effects of capillarity. The justice of this criticism is confirmed by 

 the fact that M. Plantamour found that two levels only two meters 

 apart did not give perfectly accordant results. Levels are moreover, 

 perhaps, scarcely sensitive enough for an examination of the smaller 

 oscillations of level. Dr. Siemens' form of level possesses ample sen- 

 sibility, but is probably open to the same objections on the score of 

 capillarity. 



In the case of our own experiments we think that the immersion of the 

 whole instrument in water from top to bottom has proved an excellent 

 precaution against the effects of change of temperature, and our experience 

 leads us to think that much of the agitation of the pendulum in the 

 earlier set of experiments was due to small variations of temperature 

 against which we are now guarded. 



The sensitiveness of the instrument leaves nothing to be desired, and 

 were such a thing as a firna foundation attainable, we could measure the 

 horizontal component of the lunar attraction to a considerable degree of 

 accuracy. "We believe that this is the first instrument in which the vis- 

 cosity of fluids has been used as a means of eliminating the effects of 

 local tremors. In this respect we have been successful, for we find 

 that jumping or stamping in the room itself produces no agitation of the 

 pendulum, or at least none of which we can feel quite sure. We are 

 inclined to try the effect of fluids of greater viscosity, such as glycerine, 

 syrup of sugar, or paraffin oil. But along with such fluids we shall 

 almost inevitably introduce air-bubbles, which it may be hard to get rid 

 of. If a fluid of great viscosity were used, we should then only observe 

 the oscillations of level of periods extending over perhaps a quarter to 

 half a minute. The oscillations of shorter periods are, however, so 

 inextricably mixed up with those produced by carriages and railway 

 trains, that nothing would be lost by this. 



In connection with this point Mr. Christie writes to me, that ' In the 

 old times of Greenwich Fair, some twenty years ago, when crowds of 

 people used to run down the hill, I find the observers could not take 

 reflection observations for two or three hours after the crowd had been 



turned out We do not have anything like such crowds now, even 



on Bank holidays, and I have not heard lately of any interference with 

 the observations.' If the observers attributed the agitation of the mer- 

 cury to the true cause, the elasticity of the soil must be far more perfect 

 than is generally supposed. It would be surprising to find a mass of 

 glass or steel continuing to vibrate for as long as two hours after the 

 disturbance was removed. May it not be suspected that times of agita- 



