92 APPENDIX. 



the horizontal and vertical distances through which they have travelled should 

 be noted. 



The effects of an earthquake in altering the superficial features of the ground should 

 be recorded ; but here care must be taken not to confound such a purely secondary cause 

 as that to which the sinkings and fissurings of the river banks in the Cachar earth- 

 quake of 1869 were due, with actual alteration of level accompanying the earthquake. 



We now come to such observations as can only be made with the aid of instru- 

 ments, of which the simplest, and one which may often be of considerable service, is — 



(1) A round tub or basin half filled with water, which should be coloured — a little ink 

 answers as well as anything else — while the sides of the tub are whitened; when the 

 shock comes, the water surges up on that side from which the shock is travelling and 

 leaves a mark. 



(2) An elastic rod firmly fixed in the ground, with a heavy weight, such as a 

 spherical 4-pound shot, at the top ; the free portion of the rod, which may be of stout 

 rattan, should be the length of a second's pendulum, and the spherical weight must be 

 surrounded by a wooden hoop, leaving an annular space of 6" or 8" in breadth between 

 it and the surface of the ball. In this, eight holes are to be bored and fitted with 

 cylindrical rods of wood, such as lead-pencils, sliding easily in their respective holes ; 

 these rods to face the cardinal points and their bisections. 



(3) The best of all pendulum apparati is that suggested by Mr. E. Mallet, which 

 consists of four inverted pendula, with broad, thin, flexible, supports, capable of vibrating 

 only in one direction and placed facing the four cardinal points of the compass. Each 

 of these is provided with a ratchet and pawl, which allows of its bending over in one 

 direction only, and retains it in position when so bent. 



Besides the above, several other forms have been suggested, all considerably 

 complicated ; but as they all suffer from the drawbacks inherent in any seismometer 

 which depends on the principle of the pendulum, it would serve no useful purpose to 

 describe them : we will therefore pass on to a consideration of the advantages or defects 

 of those already described : — 



(1) The water in the circular basin is, theoretically, set in motion in a single 

 plane, but practically the unequal friction with the sides gives it a swirling motion, 

 which causes it to surge round the basin, often making it a very difficult matter 

 to decide where the mark reaches highest, and consequently from whence the 

 shock came. When, as is the case in most violent shocks, the main wave is accom- 

 panied by minor oscillations, more or less transverse to it, this swirling is very 

 much exaggerated. There is auother drawback ; in that, under the action of violent 

 shocks, the water first rises against the side from which the shock came, but is then 

 dragged forwards and thrown up against the opposite side of the basin, and if the 

 period of oscillation of the water in the basin at all corresponds with that of the earth- 

 quake wave, it will rise highest on that side towards which the earthquake is travelling. 

 Notwithstanding these drawbacks, the ease with which this instrument can be impro- 

 ( 92 ) 



