454 



SCIENCE 



[N. S. Vol. XXXIV. No. 875 



No one appears to have considered the effect 

 of oceanic precipitation in the middle geologic 

 ages. 



It is sufficient for this discussion to divide 

 the earth's geologic history into three periods; 

 the fluid age, the crust-steam age, and the 

 crust-ocean age. During the crust-steam age 

 the crust increased gradually from nothing at 

 all to an efl'ective thickness, including the 

 layer of partial solidification, of several miles. 

 During the larger part of this age, the water 

 now forming the oceans was probably all in 

 the atmosphere. The period of oceanic pre- 

 cipitation was probably quite extended on ac- 

 count of the heat liberated by condensation 

 as well as the increased admission of solar 

 radiation due to the clearing of the atmos- 

 phere. 



At the time when liquid water began to ex- 

 ist in quantity, the cooling of the earth's 

 crust had progressed far below the freezing- 

 points of nearly all the materials of the crust, 

 and the most plausible assumption is that the 

 crust was of considerable thickness. It is 

 also probable that the earth's surface was 

 fairly level, the flatness depending upon the 

 uniformity of distribution of rock materials 

 of different densities. 



Consider now the effect of superposing on 

 this early crust a great quantity of material 

 of low density, namely, the water of the oceans. 

 Near the borders of the great oceans, we 

 should then expect to find a severe outward 

 thrust of crust comparable in mass to the 

 mass of the ocean, but in volume less in in- 

 verse proportion to its density. 



The isostatie conditions would further 

 lead us to look for the accommodation to this 

 displacement material farther inland, in a 

 general continental elevation increasing 

 toward and up to the border ranges. 



This suggestion is put forward for what it 

 may prove to be worth. Whatever may have 

 been its consequences in detail, if the isostatie 

 condition has been even approximately ad- 

 hered to during the earth's history, the pre- 

 cipitation of the oceans must have had a pro- 

 found effect on the elevation and depression 

 of portions of the earth's crust. With more 



definite knowledge of the physical properties 

 of geological materials it should be a mere 

 mathematical problem to determine what 

 those effects have been. 



p. g. notting 

 Washington, D. C, 

 September 12, 1911 



MUSICAL ECHOES 



The phenomenon of musical echoes has been 

 known for a long time and has secured recog- 

 nition in the text-books on sound. Thus we 

 read :' 



Frequently, a sharp sound, such as the clapping 

 of the hands or of two boards together, is reflected 

 in a room or a corridor with smooth, parallel walla 

 as a more or less musical sound. A similar effect 

 is often observed when one is walking near pali- 

 sading, each footstep of the observer being fol- 

 lowed by a musical ring. The effect is only noted 

 after some sudden sound, and may often be heard 

 very distinctly on clapping the hands or on knock- 

 ing two stones together. 



The Greek Theater at the University of 

 California presents a pronounced musical 

 echo, the conditions being especially favorable 

 to the production of the phenomenon. The 

 seats are made up of a series of large concrete 

 steps that are semicircular in shape and that 

 rise regularly towards the back. If an ob- 

 seryer generates a sharp sound in front of the 

 stage at the center of the circles of steps the 

 sound passes out symmetrically and strikes 

 the steps in perpendicular planes and is re- 

 flected and diffracted back to the source of 

 sound. The pulses of sound reflected from the 

 successive steps follow each other regularly 

 and thus set up a musical sound which is 

 heard by the observer. 



It occurred to the author that the pitch of 

 the sound might be determined. The method 

 of experiment was to generate a musical 

 echo as already described and to compare the 

 pitch of this sound with that of an adjustable 

 turning fork. A check on the final result was 

 found by calculating the pitch from the rela- 

 tion n = v -^\ where n is the pitch of the 

 sound, A the wave-length and v the velocity of 

 sound at the temperature of the theater. It is 



•Poynting and Thomson, "Sound," pp. 31, 32. 



