EAETH. 



EDUCATION. 



395 



from that of figure. Either, now, the axis 

 keeping its place will be no longer a principal 

 axis, or following the receding centre of grav- 

 ity, the figure of the globe must change corre- 

 spondingly, to allow of rotation about the new 

 direction. The rigid materials of tho earth 

 forbid any gradual accommodation of the form ; 

 and the axis of rotation remains fixed; but the 

 constraint thus imposed by the earth's rigidity 

 on the axis occasion#a pressure tending con- 

 tinually to restore the axis to the position in 

 which it will be a principal axis, so that the 

 materials of the globe may revolve with bal- 

 anced forces about it. This tendency accumu- 

 lating, must in the course of ages exceed the 

 force of rigidity, which is a constant quantity, 

 and the axis breaking away suddenly must be 

 restored to the direction through the centre 

 of gravity, becoming anew a principal axis. 

 The accomplishing of such change must be at- 

 tended with those fearful convulsions, disloca- 

 tion of strata or bodies of land, and cataclysms, 

 which mark the boundaries of geological 

 epochs : and which, at remote periods, change 

 the face of the continents, alter the distribution 

 of heat and cold, upheave mountain chains, and 

 overwhelm whole tribes of organized beings in 

 a common and general destruction. The au- 

 thor supposes that minuter displacements of 

 the axis may go on, or suddenly occur, at other 

 than these great epochs of change ; and that 

 earthquakes are perhaps the disturbances due 

 to convulsive efforts of the revolving globe to 

 throw its actual axis, which is not one of 

 equilibrium, into the place of a principal axis, 

 which would be such. It must be said at this 

 point, that a careful consideration of the actual 

 phenomena attending earthquakes, and of their 

 connection with volcanic eruptions, will suggest 

 many difficulties in regard to this part of the 

 theory. Mr. Ogilby declares, however, that a 

 comparison of the observations of Eratosthenes 

 with those of the present time, after all due 

 allowance for variation of obliquity of the 

 earth's axis to that of the ecliptic and for 

 errors of observation is made, proves that the 

 latitude of Syene has increased within the last 

 2,100 years to the amount of 17' 21".5 ; and he 

 intimates that the astronomer royal has recent- 

 ly detected a slight annual motion of the pole 

 very nearly corresponding in amount to this. 



Mr. Robert Mallet, 0. E., whose researches 

 during a few years past into the phenomena 

 of earthquakes have well nigh established the 

 claims of Seismology to be regarded as a science, 

 laid before the Royal Society, May 8, 1862, a 

 sequel to his "Reports on Earthquake-Wave 

 Experiments," and in which he gave the results 

 for certain species of rocks, of investigations 

 into the Velocity of Earthquake Waves. He 

 first determined the ratio (modulus) of elas- 

 ticity, separately, of slate and quartz rocks, 

 hard and soft, and in directions both parallel 

 and transverse to the direction in which the 

 laminae (or the strata) of those rocks are placed : 

 this, of course, involving 8 separate series of 



experiments, and in which the observations 

 were made at intervals of pressure increasing 

 continually by 1,000 Ibs. He finds that gen- 

 erally quartz rock is less compressible than 

 slate ; while, on the contrary, the softest quartz 

 is much more compressible than the softest 

 slate, when the pressure occurs in both in the 

 direction parallel to their Iamina3. In the like 

 direction, the hardest slate is more than twice 

 as compressible as the hardest quartz. In the 

 direction transverse to the laminse, slate and 

 quartz, whether soft or hard, have in each 

 condition very nearly the same compressibility ; 

 but in this direction, the compressions of the 

 softest are about 4 times those of the hardest 

 of these rocks. The great compressibility par- 

 allel to the laminse appears to arise chiefly 

 from the fact that the mass of the rock is made 

 up of minute wedge-shaped mineral particles, 

 deposited with their largest dimensions lying 

 mainly in the direction of the plane of the lam- 

 inae, and so acting upon each other like so 

 many wedges. The wave transmitted trans- 

 versely to the laminaa will, if the rocks are 

 equally solid and continuous both ways, be 

 the greater; but breaks and discontinuity of 

 any sort must retard its advance. Mr. Mallet 

 deduces mathematically the conclusions that, 

 were the rocks perfectly solid and continuous, 

 the mean velocity of wave transmission for 

 slate and quartz would be transverse to the 

 lamina?, 13,715 ft. per second; and parallel 

 with the lamin, 7,659 ft. per second. In 

 nature, however, owing to the greater discon- 

 tinuity of the rocks in the former direction, the 

 relative velocities of waves in the two direc- 

 tions must be nearly the reverse of these. The 

 results, though apparently disagreeing with 

 those of Helmholtz and others in reference to 

 compressibility and elasticity of wood in the 

 three principal directions of section (see WOOD, 

 New Amer. Cyclopedia), do not so disagree in 

 reality. In case of an earthquake in Italy, 

 Dec. 16, 1857, the phenomena of which Mallet 

 has thoroughly investigated, and the primitive 

 shock of which was delivered in an upward 

 line (the seismic vertical), piercing the village 

 of Caggiano, in the valley of Salaris, the actual 

 velocity of the wave not far from this point 

 was found to be 700-800 ft. per second ; the 

 velocity of any single wave-particle being 13-14 

 ft. per second. These velocities diminish with 

 increasing distance from the central point of 

 impulsion, until they disappear in parts where 

 the wave ceases to be propagated. 



EDUCATION. The diffusion of education 

 in the United States has been greater than in 

 any other country of the world, excepting per- 

 haps the kingdom of Prussia ; but the expen- 

 diture for educational purposes is at the present 

 time greater than in any other country. The 

 instruction imparted has been chiefly elemen- 

 tary in its character, the larger part of the 

 population being taught to read and write, and 

 acquiring a knowledge of the elements of arith- 

 metic, geography, history, and language. Yet 



