MEASUREMENT OF EARTHQUAKE-WAVES. 587 



close to the well-known " Hermitage," or half-way house, in the ascent 

 of the mountain. Being raised on this ridge above the surrounding 

 country, it is comparatively safe from the molten lava that flows at 

 times on either side of it. 



The building itself is handsome ; in fact, it is to be regretted that 

 so much money should have been devoted to the masonry instead of 

 to additional instruments. On the ground-floor are the inhabited 

 rooms, all scantily furnished ; but the pursuers of science cannot al- 

 ways expect bodily comfort. On the first floor we find the Museum, 

 with a fine collection of minerals found on the mountain. Perhaps it 

 may be as well here to correct the common mistake as to the nature of 

 the yellow substance found about the craters, whose brilliant colors re- 

 mind one so much of the Solfatara. This substance is not sulphur, but 

 copper. The most interesting objects in the Museum are the "funie- 

 rolles," or smoke-holes. Occasionally at the end of an eruption you 

 may see at the bottom of the crater a small cone of lava, with a hole 

 in its top, through which the steam pours with a hissing noise like a 

 wave breaking on a pebbly beach, or like a blast-furnace, or, as Pliny 

 has it, like the grinding of a saw ; the intensity of the sound varying 

 with your position. These small cones are the fumerolles ; they are a 

 foot or two high ; and Palmieri has actually had several of these natu- 

 ral chimneys cut off and transported to the Museum. 



We now pass on to the Observing-Room. There are solid piers 

 carried up from the ground to support the instruments. First comes 

 the elegant seismograph, an instrument for the automatic registration 

 of earthquake-shocks. The object of the instrument is twofold : first, 

 to measure the direction and intensity of a shock; and, second, to 

 write down a history of the earthquake. The shock may be either 

 vertical or horizontal, or partly vertical and partly horizontal. For 

 the vertical shocks a fine metallic point is suspended by a coil of wire 

 over a cup of mercury. The coil of wire acts as a spring, and the 

 slightest upward motion of the earth is sufficient to cause the point to 

 dip into the cup of mercury. This completes a galvanic circuit, which 

 stops a clock at the exact half-second at which the shock occurred, 

 and rings a bell to call the observer, and also does other work which 

 we shall speak of again. There are three or four helices of wire of 

 different strengths, which support small magnets above a cup of iron 

 filings. When a vertical shock occurs, some of these magnets dip into 

 the iron filings. To one of these a light index is attached, for measur- 

 ing the intensity of the shock. 



For horizontal shocks there are four glass tubes. Each of them is 

 bent twice at right angles, so as to form a U-tube. One arm of this 

 tube has more than double the diameter of the other, and is shorter. 

 The four tubes point in the directions of the four cardinal points. 

 Each tube has a certain quantity of mercury poured into it, and on the 

 surface of the mercury, within the narrow arm of the tube, there rests 



