Nature . already indicated, withobservation. The result at which Knott arrives indicates that the square ofthe speed increases at o9 per cent, per mile of descent in theearth, the formula being z;- = 29 -t- 026 (/ in mile second units. With an initial velocity of I 7 miles per second, the velocitiesat depths of 400, 800, 1200 . . 4000 miles become 37, 4-9,5-8, 67, 7-4, 8-1, 87, 9-3, 9-8, and 10-3 miles per second. Thetimes taken for wave-fronts to reach the positions shown areindicated in the sketch ; the time taken to pass through theearth being 22 minutes. I assume that when a wave has passe
Nature . already indicated, withobservation. The result at which Knott arrives indicates that the square ofthe speed increases at o9 per cent, per mile of descent in theearth, the formula being z;- = 29 -t- 026 (/ in mile second units. With an initial velocity of I 7 miles per second, the velocitiesat depths of 400, 800, 1200 . . 4000 miles become 37, 4-9,5-8, 67, 7-4, 8-1, 87, 9-3, 9-8, and 10-3 miles per second. Thetimes taken for wave-fronts to reach the positions shown areindicated in the sketch ; the time taken to pass through theearth being 22 minutes. I assume that when a wave has passed from its origin beyondthe region vaguely referred to as the crust of our earth, it thenspreads in all directions through a mass in which there is only anextremely gradual change in elasticity and density with regard toits centre. All wave-paths, however, before they emerge at thesurface, encounter at varying obliquities the under-surface ofthis crust. For purposes of illustration, we will assume this. region of abrupt change to lie on the 400-mile circle. Thepath /i meets this surface nearly at right angles, whilst p., p-^meet it at decreasing angles less than right angles. After eachof these incidences a condensational wave will be refracted andsplit up into condensational and distortional rays. Now it willbe observed that these two waves, which I will call < and d,will have different distances to travel before actual emergence,which distances will increase from /, towards /j. At any stationP, the first arrival will be c, but as this will be eclipsed on thearrival of d, its duration will always be short, and unless theoriginating shocks are well separated, seismograms, as we know-them, can never show more than one set of condensationaltremors. At some point, like p.^, the duration of the preliminarytremors should reach a maximum, but from this point on to-wards the origin this quantity will decrease, if only on accountof the fact that the velocity along the brachistrochro
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