5 8b THE POPULAR SCIENCE MONTHLY. 



to those of which Torula is a sort of offshoot. It is certain that such dis- 

 eases are propagated by contagion and infection, in just the same way 

 as ordinary contagious and infectious diseases are propagated. Of 

 course, it does not follow from this, that all contagious and infectious 

 diseases are caused by organisms of as definite and independent a char- 

 acter as the Torula y but, I think, it does follow that it is prudent and 

 wise to satisfy one's self in each particular case, that the " germ-theory " 

 cannot and will not explain the facts, before having recourse to hy- 

 potheses which have no equal support from analogy. Contemporary 

 Review. 



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MEASUREMENT OF EAKTHQUAKE -WAVES. 



By GEOEGE FOEBES. 



WHILE the scientific world and his own countrymen are rivals in 

 doing honor to Prof. Palmieri for his zeal in remaining at his 

 post in spite of all danger, it may be interesting to examine in some de- 

 tail the work done at the Observatory of Mount Vesuvius. We know 

 wonderfully little about the origin and mutual dependence of volcanic 

 phenomena. This is due to a want of accurate observations. For the 

 complete investigation we require first to know at what dates earth- 

 quakes and eruptions occur at different parts of the earth. Next we 

 must have observations of the direction and exact hour at which a 

 wave of disturbance passes different places whose positions are known. 

 This gives us the velocity of the wave, and helps to determine the 

 position, under the earth's surface, of the centre of disturbance ; or, if 

 a wave be propagated over the sea, we obtain a means of estimating 

 the average depth of the intervening ocean ; for the velocity of a wave 

 increases with the depth of the sea. This method gives one of the 

 best determinations we possess of the depth of the Pacific Ocean. But 

 beyond this we must have observations made systematically at some 

 place subject to earthquakes and volcanic eruptions. No place in 

 Europe is more suitable for this than the neighborhood of Mount 

 Vesuvius ; and it was for such observations that an observatory was 

 established there. 



Every one knows that Mount Vesuvius consists of a vast cone of 

 lava and ashes, at the top of which is the great crater. On the north- 

 ern side, separated from it by the deep valley called the Atrio del Ca- 

 vallo, rises the precipitous and semicircular Monte Somma. This once 

 formed the crater of the volcano, and the present cone seems to have 

 been formed inside that great crater at the time when Pompeii was 

 overwhelmed. On a spur of rock, a mile or two in length, running 

 down from the Atrio del Cavallo, the Observatory is placed. It ia 



