Page:Popular Science Monthly Volume 64.djvu/234

230 there was no means of judging forward motion. The discharge from a cannon would 'roll, squirt and seethe' without showing any forward motion, if we were looking into the cannon's mouth. There can be no question, however, that there was an avalanche of solid materials which accompanied the second black cloud seen by Anderson and Flett, and I am tempted to ask, may there not have been a similar non-incandescent avalanche with the first cloud, representing the relatively cool material which cleared the crater's throat?

If this were true we should have with each puff a sequence of events somewhat as follows:

 1. Straight puff upwards. 2. Avalanche of heavy materials downwards. 3. Horizontal dust-cloud outwards.

The sharply differentiated horizontal and vertical blasts had been noted in all the eruptions of these volcanoes. They may be seen in the photograph of the eruption of July 16, where one cloud lowers over the water, the great column is upright, and its crest is bent to the east by the high counter trade-wind. (Fig. 1.)

The three stages above mentioned, in an explosive discharge from a fissure upward, might have been expected on purely a priori grounds.