MICHIGAN ACADEMY Ol' SCIENCE. 27 



Thus becoming interested in ballistics, lie made a thorough investigation 

 of the strains developed by explosions in the chambers of guns, and a 

 monograph upon the subject which lie published in 1856 brought him 

 general recognition and many honors. A year later, in 1857, occurred 

 the great earthquake in the Basilicata, generally referred to as the 

 "Great Neapolitan Earthquake," since the district was included in the 

 former kingdom of Naples. Believing himself by reason of his studies 

 of explosives to be specially fitted to investigate this disturbance, Mallet 

 applied to the Royal Society for a grant of money, and his request being 

 approved, he visited the Jiasilicata, and in 1862 in two sumptuous volumes 

 the earthquake was explained as the result of an explosion that had 

 occurred in a cavity beneath the region affected, the damage upon the 

 surface being explained under the laws of transmission of stress such 

 as had applied in his earlier researches upon cannon. No one familiar 

 with these circumstances can reasonably doubt that the trend of his 

 theory was already determined by his life history before his sailing 

 from England. It should be added that through giving to the problem 

 of earthquakes a mathematical treatment, Mallet's study had the effect 

 of removing seismology from the field of geology for the jx-riod of nearly 

 half a century, and giving it over to the elasticians. 



It lias not been altogether uncommon for students of science, and even 

 those of the highest rank, when drawing conclusions, to fail to take 

 proper account of the fact that the observed rates of change, or gradients, 

 perhaps of temperature or pressure, which they have been able to 

 verify for limited distances only, may not be assumed to continue in- 

 definitely without interruption or variation. No less distinguished a 

 physicist than Helmholtz on the basis of the known aero-thermic gradient 

 as determined over Europe to a height of about nine kilometers, declared 

 that the atmosphere could not extend beyond twenty-seven or twenty- 

 eight kilometers from the earth's Surface, since the absolute zero of 

 temperature would be reached at that level. Sounding of the atmosphere 

 has since been carried to fully twice the limit fixed by Helmholtz, and 

 has revealed the fact that a few kilometers beyond the limit of explora- 

 tion when he made his prediction, or at an average altitude of eleven 

 kilometers, the aero-thermic gradient is interrupted and succeeded by 

 practically isothermal conditions above. 



Today in reputable treatises one may read in round numbers the sup- 

 posed temperature at the center of the earth, and based upon what.^ The 

 geothermic gradient determined for the shell of rock immediately beneath 

 the earth's surface and verified roughly for about one four-tliousandth 

 of the entire distance to the earth's center. Can we assume that our 

 yard-stick in this instance is suitable for measurement tliroughout the 



