J. D. Dana — History of the Changes in Kilauea. 19 



The diminution since 1823 in the height of the western wall 

 above the black ledge is probably due almost wholly to the 

 'flooding of the black ledge. According to the numbers, this 

 diminution was about 185 feet from 1823 to 1832 ; 65 from 

 1832 to 1840; and 160 feet since 1840. But subsequent to 

 1840, as Emerson's map shows, the diminution of level along 

 the black ledge or lateral portion of the pit has been much less 

 than over the central, the amount of diminution at center having 

 been at least 200 feet, and about Halema'uma'u 250 to 300 feet. 



The bottom of the emptied basin of Halema'uma'u after the 

 eruption of 1886 was 900 feet below the Volcano House : and 

 this was 50 to 100 feet above the liquid lava of the basin in 1840. 



The relations oetween the amounts discharged in 1823, 1832, 

 1840 and 1868 could be approximately inferred from the size 

 of the lower pit as determined by the mean breadth of the 

 black ledge, if the width of the crater were the same at all pe- 

 riods. But in addition to other uncertainties we have that 

 arising from sloping walls, and very sloping on the southeast 

 side. The pit of 1823 should therefore have been narrower at 

 the black-ledge level than that of 1840. Still, the width of 

 the ledge in 1823, according to all the observations and maps, 

 was so very narrow compared with that in 1840, that we may 

 feel sure of the far larger amount of the earlier discharge. But 

 the depth of the lower pit was also greater in 1823, and this 

 requires an addition of one half to the amount which the area 

 of the lower pit suggests, if not a doubling of it. 



For an estimation of the discharge of 1832 we are still more 

 uncertain as to the mean width of the ledge. But that the 

 ledge was narrow, much like that of 1823, is most probable. 

 In 1868 the down-plunge, according to the most reliable esti- 

 mate, was a fourth less than in 1840, the depth of the pit being 

 not over 300 feet. 



There are no sufficient data for putting in figures the rela- 

 tive amounts of discharge at the great eruptions. But the 

 general fact of a large diminution in the amounts since the first 

 in 1823 is beyond question. It has to be admitted, however, 

 that we can hardly estimate safely the discharge in 1868 from 

 the size of the pit then made, since the thickness of the solid 

 floor of the crater may have prevented as large a collapse in 

 proportion to the discharge. But it did not take place until 

 28 years had passed after 1840, and this strengthens the evi- 

 dence as to an apparent decline in the outflows, whatever be 

 true as to the activity. The following eighteen years produced 

 only minor eruptions. 



4. Other points in the Topographic history of the Kilauea region. 

 Besides the points considered, the chief events in the topo- 

 graphic history since 1823 are : (1) avalanches and subsidences 



