The ancient rivers had also carried gold to the valleys, and to collect this a dredge, which the miners called a "gold ship," came into use. The "ship" part of this machine is an immense flat scow. Stretching out from one end is something which looks like a moving ladder. This is the support of an endless chain of buckets, each of which can bite into the gravel and take a mouthful of five or six hundred pounds. They drop this gravel into a big drum which is continually revolving. Water flows through the drum, and washes out the sand and bits of gold over large tables, where by means of riffles and quicksilver the gold is captured. This scow was usually on dry land at first; but its digging soon made a lake, and then it floated. It must be more fascinating to hold a pan in your own hands and pick out little grains of gold or perhaps even a big piece of it with your own fingers, but if the gravel is good the dredge makes more money. In Alaska the great difficulty in mining is that, except at the surface, the ground is frozen all the year round. At first, the miners used to thaw the place where they wished to dig by building wood fires; but this was a slow method, and now the thawing is done by steam. They carry the steam in a pipe to the place where the digging is to be done, and send it through a hose. At the end of the hose is a pointed steel tube. They hammer this tube into the ground and let some steam pass through the nozzle. This softens the ground so that picks and shovels may be used. There is generally cold enough in Alaska, but once at least the miners had to manufacture it. The gold-bearing gravel was deep, the ground was flat, and it was often overflowed. They set up a freezing plant, and shut in their land with a bulkhead of ice several feet thick. Then they pumped out what water was already in and did their work with no more trouble. When gold began to grow less in the California gravel, the miners looked for it in the rocks on the mountain-side. The placer miners laughed at them and called their shafts "coyote holes"; but in time the placers failed, while nearly all of our gold to-day comes from veins of white quartz in the rocks. A vein of gold is the most capricious thing in the world. It may be so tiny that it can hardly be seen, then widen and grow rich in gold, then suddenly come to an end. This is why a new mine is so uncertain an enterprise. The gold may hold out and bring fortunes to the investors, or it may fail, and then all they will have to show for their money is the memory that they put it into a hole in the ground. The managers of a few of the well-established mines, however, have explored so far as to make sure that there is gold enough for many years of digging.