EARTHQUAKE PHENOMENA. 417 



rial, of about eight inches diameter; bore four smooth holes through 

 the hoop in the plane of its circle, and at points ninety degrees dis- 

 tant from each other; adjust through each of these a smooth round 

 rod of wood, (an uncut pencil will do well.) and make' them, by 

 greasing, &c, slide freely, but with slight friction, through the holes. 

 Secure the hoop horizontally at the level of the centre of the ball by 

 struts from the block, and the ball being in the middle of the hoop, 

 slide in the four rods through the hoop until just in contact with the ball. 

 It is now obvious that a shock, causing the ball to oscillate in any 

 direction, will move one or more of the rods through the holes in the 

 hoop, and that they will remain to mark the amount of oscillation. 

 A similar apparatus, with the pendulum rod secured horizontally, 

 (wedged into the face of a stout low wall, for example,) will give the 

 vertical element of the wave. Two of these should be arranged — 

 one north and south, the other east and west. One objection to this 

 and all apparatus upon the same principle is, that as the centre of 

 elastic effort of the pendulum rod never can be insured perfect!} 7 in 

 the plane passing through the centre of gravity of the ball for every 

 possible plane of vibration, so an impulse in a single plane produces 

 a conical vibration of the pendulum, and hence the ball deranges the 

 position more or less of the index rods which are out of the true 

 direction of shock. Moving the apparatus by hand, and a little prac- 

 tice in observation of its action, will, however, soon enable a pretty 

 accurate conclusion as to the true line of shock to be deduced from it. 

 It will be manifest that the observer must record minutely the 

 dimensions and other conditions of such apparatus, where not per- 

 manently kept, to enable calculations as to the wave of scientific 

 value to be made from his observations of the range of either fluid 

 or solid pendulums. 



A common bowl partly filled with a viscid fluid, such as molasses, 

 which, on being thrown by oscillation up the side of the bowl, shall 

 leave a trace of the outline of its surface, has been often proposed 

 as a seismometer. This method has many objections; it can only 

 give a rude approximation to the direction of the horizontal element; 

 but as it is easily used, should never be neglected as a check on 

 other instruments. A common cylindrical wooden tub, with the sides 

 rubbed with dry chalk, and then carefully half filled with water or 

 dye-stuff, would probably be the best modification. 



Another extemporaneous instrument for measurement of vertical 

 motion in the wave may be sometimes useful. Make a spiral spring 

 of eighteen inches or so in length by twisting an iron wire of one- 

 eighth of an inch diameter round a rod of about 1| inch diameter, 

 (the staff of a boarding-pike ;) suspend it by one end vertically from 

 a fixed point, and fix a weight (a twelve-pound shot will do) to the 

 lower end, and below and in a line passing vertically through the 

 centre of gravity of the weight fix the stem of a common tobacco 

 pipe; let the lower end of this stem just dip into a deep cup filled 

 with pretty thick common ink or other colored fluid; the action of 

 this needs no description. 



The preceding instruments suffice at once to give the direction of 

 transit of the earth- wave and its dimensions; its rate of progress or 

 9.7 



