ON THE MEASUKEMENT OF THE LUNAK DISTURBANCE OF GRAVITY. 125 



tion, such as those noted by M. d'Abbadie, happened to coincide on two 

 or three occasions with Greenwich Fair ? 



As the sensitiveness of our present instrument is very great, although 

 the sensitising process has never been pushed as far as possible, we think 

 that it will be advantageous to construct an instrument on half, or even 

 less than half, the present scale. The heavy weights which we now 

 have to employ will thus be reduced to one-eighth of the present amount. 

 The erection of the instrument may thus be made an easy matter, and 

 an easily portable and inexpensive instrument may be obtained. 



Our present form of instrument has several serious flaws. The image 

 is continually travelling off the scale, the gearing both internal and ex- 

 ternal to the room for observing is necessarily complex and troublesome 

 to erect, and lastly we have not yet succeeded in an accurate determina- 

 tion of the value of the scale. 



"We are in hopes of being able to overcome all these objections. We 

 propose to have a fixed light, which may be cast into the room from the 

 outside. This Avill free us from the obviously objectionable plan of having 

 a gas-flame in the room, and at the same time will abolish the gearing 

 for traversing the lamp on the scale. We should then abolish the dis- 

 turbing pendulum and thus greatly simplify the instrument. The read- 

 ings would be taken by the elevation or depression of the back-leg, until 

 the image of the fixed light was brought to the cross wire of the observing 

 telescope. 



The ease with which the image may be governed with our present 

 arrangements leads ua to be hopeful of the proposed plan. The use 

 of the back-leg will, of course, give all the displacements in absolute 

 measure. 



The only gearings which it will be necessary to bring outside the 

 room will be those for sensitising and for working the back-leg. The 

 sensitising gearing when once in order will not have to be touched 

 again. 



The objections to this plan are, that it is necessary to bring one of the 

 supports of the instrument under very slight stresses, and that it will not 

 be possible to take readings at small intervals of time, especially if a more 

 viscous fluid be used. 



Our intention is to proceed with our observations with the present 

 instrument for some time longer, and to note whether the general 

 behaviour of the pendulum has any intimate connection with the 

 meteorological conditions. We intend to observe whether there is a 

 connection between the degree of agitation of the pendulum and the 

 occuiTence of magnetic storms. M. ZoUner has thrown out a suggestion 

 for this sort of observation, but we find no notice of his having acted 

 on it.' 



We shall also test how far the operation by means of the back-leg 

 may be made to satisfy our expectations. 



We have no hope of being able to observe the lunar attraction in the 

 present site of observation, but we think it possible that we may devise 

 a portable instrument, which shall be amply sensitive enough for such a 

 purpose, if the bottom of a deep mine should be found to give a suffi- 

 ciently invariable support for the instrument. 



The reader will understand that it is not easy to do justice to an 



> PUl. Mag. Dec' 1872, p. 497. 



