298 Mr. E. Maliefc 072 the Seismic Results obtained 



dissociation furnishes another ilkisli-ation of a simihir character. 

 Deville {Com2ites Hendus, Lxxxiv. p. 1259) says: — '' My first 

 experiments on the dissociation of water, in perfect accord 

 with those which M. liunsen has since pubKshed, demonstrate 

 that at temperatures of 2500°, according to M. Debray and 

 myself, or of 2800°, according to the ilkistrious professor of 

 Heidelberg, nearly half of this vapour is reduced to its ele- 

 ments, hydrogen and oxygen." The ratio 2800° : 100° agrees 

 precisely with a very probable estimate of the ratio between 

 solar and terrestrial superficial attraction. 



XXXIX. Oil the Seismic Results obtained from the Hallet^s- 

 Point Uxjjlosion. 



To the Editors of the Pliilo sop] deal Magazine and Jonrnal. 



Gentlemen, 

 ^XTILL you allow me space for a few observations upon 

 V T some of the results of the great explosion of Sept. 24, 

 1876, at Hallet's Point, Xew-York Harbour, as I find them 

 recorded in a printed copy of a paper read before the National 

 Academy of Sciences of America, Oct. 18, 1876, and before 

 the Essayons Club of the Corps of Engineers, jSTov. 14, 1876 

 (vol. xl. of printed papers). For a copy of this paper by 

 Major-General Abbot, U.S. Engineers, I believe I have been 

 indebted to that officer, who transmitted it through my son, 

 Dr. J. W. Mallet, Professor of Chemistry, University of Vir- 

 ginia, some time since, though I have but recently been able 

 to make myself acquainted with its contents. While the pri- 

 mary object of this great explosion was the engineering one 

 of destroying the reef, which appears to have been most suc- 

 cessfully accomplished, a subsidiary object of a purely scientific 

 character was sought to be attained, viz. the determination of 

 the transit-velocity, or rate of progression, of the elastic wave 

 of shock through those superficial portions of the earth's crust 

 adjacent to the seat of explosion. I cannot but regret that 

 this experiment was ever attempted, as the circumstances of 

 the case are so ill suited to the determination of so deli- 

 cate a physical question as, in my judgment, to be rather 

 calculated to retard than advance our knowledge of observa- 

 tional seismology. I shall not presume to occupy your space 

 by recurring to the details of the arrangements of the explo- 

 sion, but refer such of your readers as take an interest in the 

 subject to Major-General Abbot's paper, and proceed at once 

 to remark: — 



It is of primary importance to any accurate determination of 



