138 



REPORT — 1897. 



numerous. The foci of submarine disturbances — such, for example, as 

 those which from time to time have interfered with telegraph cables — 

 may possibly be determined, and new light thrown upon changes taking 

 place in ocean beds. The records throw light upon certain classes of dis- 

 turbances now and then noted in magnetometers and other instruments 

 susceptible to slight movements, whilst local changes in level, some of 

 which may have a diurnal character, may, under certain conditions, become 

 apparent. 



The Instrument. — The general features of a type of instrument which 

 the Committee have selected as being sufficient for the attainment of 

 the objects in view are shown in the accompanying sketch. 



The instrument consists of an iron bed-plate and stand carried on 



Fig. ]. 



Pivot on Boom 



E 



^ 



oWatch 



Stand 



Boom 



P 9 O 



three levelling screws. Resting against a needle-point or pivot projecting 

 from the base of the stand, and held in a nearly horizontal position by a 

 tie, is a light aluminium boom. Attached to the outer end of this boom 

 there is a small rectangular plate in which there are two slits, one of which 

 is large and the other is small. Partly for the purpose of balancing the 

 weight of the outer end of the boom, and partly for obtaining the ' steady 

 point ' of a seismograph between the attachment of the tie to the pivot, a 

 weighted cross-bar is pivoted. 



When the boom swings to the right or left, the rectangular plate with 

 its slits passes to the right and left across a fixed slit in the lid of a box, 

 inside which a 2-inch (50 mm.) strip of bromide paper is being driven by 

 clockwork. Light from a lamp is reflected downwards by a mirror to 

 cover the whole of the latter slit. It however only enters the box to 



J 



