32 



a cone with central crater showing the peculiarities of a volcanic 

 ash cone, is gradually built up. The internal structure of such a 

 cone is here demonstrated by fixing two vertical sheets of glass 

 in a radial direction before commencing operations, and at the 

 end removing all the sawdust which lies between them. In this 

 way the inward slope of the deposit within the crater and the 

 outward slope on the flanks of the cone is fully demonstrated. 

 The proportionate height of the cone depends on the angle of 

 rest of volcanic fragments in one case and of sawdust in the 

 other, but these are not very different, hence the natural slope 

 of such cones is fairly well shown. 



For a full explanation of the structure of such cones as 

 illustrated by such a model, see Judd's " Volcanoes." 



96. Three models illustrating massive eruptions, by 

 Professor E. Eeyer of Gratz. 



Lent by the Geological Laboratory, Royal College of 

 1887. Science. 



These are produced by squeezing masses of plaster through a 

 vertical hole in the same way as that in which lava is forced up 

 through the neck of a volcano. The form it assumes is 

 dependent on its fluidity, and thus two of these models, made of 

 more fluid material, are flatter than the third, but in all cases 

 the slopes, except immediately over the aperture, are low. Each 

 of these models is cut through the middle to show the internal 

 structure, consisting of a more or less flattened circle passing 

 through the opening of the vent. The different coloured plasters, 

 red and white, show the effect of a subsequent eruption of lava 

 at the same spot. 



For full explanation, see Judd's " Volcanoes." 



97. Raised model of Mount Vesuvius, geologically 

 -coloured after Dufreynoy. 



E. 141. 1874. Prepared by J. B. Jordan. 



This is on a horizontal scale of 1 inch to the mile, and stands 

 in an oak frame 1 ft. 8 in. by 1 ft. 4 in. The various minor 

 craters on the slopes are indicated by colour, and the rest is 

 coloured geologically according as it is superficially composed of : 

 1 . Leucitic lavas ; 2. Modern lavas ; 3. Trachyte ; 4. Pumiceous 

 Tufa ; 5. Tufa of Pompeii. 



IV. SEISMOGRAPHIC APPARATUS. 



98. The Gray-Milne seismograph. 



E. 60. 1885. Made by Jas. White. 



This instrument is designed to record the motion of a point on 

 the earth's surface during an earthquake. Three rectangular 

 components of the motion are written by means of fine glass 



