APPENDIX. 97 



cubical recess must be cut, into 'which a spherical leaden ball will just fit, — a 12-bore 

 spherical bullet will do, but a larger size would be better ; now, taking the inner corner 

 of each cube as a centre and the length of its side as radius, describe a quadrant on. 

 the base of each ; up to this quadrant the wood must be bevelled, and the bevel con- 

 tinued along each longitudinal edge of the log to a point exactly 4'05, or 4 feet | inch 

 approximately, from the centre of the ball as resting in its cubical hollow, where the 

 bevel will end abruptly in a horizontal ledge ; the bevel should have been increased 

 so that this ledge shall be a quadrant of exactly similar shape to that forming the 

 base of the recess above, but of course reversed. These directions may seem rather 

 complicated, but the three sections given on plate XIV will, we fancy, render them 

 easily intelligible. On each of these lower ledges a projecting pin should be placed, 

 so that when the whole affair is set upright the pin will be vertically underneath the 

 centre of gravity of the ball above it. This post must be set absolutely vertical in 

 the ground, and dry sand be filled in up to the level of the lower ledges. To read 

 .the instrument, a straight-edge of some 6 feet in length should be made, with one 

 edge bevelled and one end cut away towards the bevelled edge, at an angle of about 

 30°. Close to this pointed end a small notch must be made on the bevelled edge, and 

 from this notch a scale of inches and tenths of an inch carefully laid off. 



After the passage of a shock it will probably be found that all four balls are dis- 

 placed, two will be found lying on the ground, more or less directly underneath their 

 original positions, while either one or both of the others have been projected to 

 some distance ; one of these should be cautiously lifted, and the point which lies 

 directly underneath the centre of the ball as it lay on the ground should be marked. 

 Then take the wooden straight-edge and place the notch against the pin, underneath 

 the original position of the ball, and bring the edge over the mark, showing where the 

 centre of the ball lay ; then read off the .direction and length of the line joining 

 these two points. Should two balls be projected, the same must be done in the case of 

 the second, and both results recorded separately. The results may be entered on the 

 same form as those of the cylinder seismometer by adding two columns, one for the 

 direction and the other for the distance to which the balls are projected. 



The height, 4*05 feet, is selected, as it is the height through which a body falls in 

 one-half of a second ; any other height would of course answer, but by taking this 

 particular value, the subsequent calculations- by which the observations are reduced 

 are very much simplified. 



It is evident that this would not be a suitable form of instrument to set up by 

 itself, as the projection of the bullets to any particular distance might be due either to 

 a high angle of emergence and a moderate velocity or to a low angle of emergence and 

 a higher velocity of shock. If, however, the velocity of motion of the horizontal com- 

 ponent of the shock be known, as would be the case approximately wherever a series 

 of cylinders was established, this instrument will give a datum from which, by a sim- 

 ple calculation, the velocity of motion in the vertical component can be obtained. 



( 97 ) 



