of rocks with which we are all acquainted--rocks like granite, or quartz, or basalt. And it will be evident to any one who thinks about the subject for a moment that no amount of pressure would make a rock as hard as a diamond. Now how have these rocks been made? The answer is that they have been made in some interior furnace of heat deep down in the earth. Sometimes they have boiled up, and we can trace them bursting their way through the sedimentary strata above them. We do not know very much about the furnaces or cauldrons whence they have come; in fact, we know very little about the depths of the earth. The deepest mine-shaft known is near Lake Superior, and is only 5000 feet in depth. In Silesia a bore-hole has been made by the Austrian Government of a mile and a quarter in depth. It would be by no means an easy task to sink a great boring. The Hon. Charles Parsons has described some of the difficulties. The shaft would have to be sunk in a neighbourhood where it would not be likely to encounter water on its way down, because otherwise there would be the necessity of pumping operations. In order to be of value for purposes of observation, the shaft would be of the size usual in ordinary mines and coal-pits. It would be sunk in stages each of about half a mile in depth, and at each stage there would be placed the hauling and other machinery for dealing with the next stage below. This machinery, in order to economise space and limit the heat of the workings, would be electrical. Even so there would have to be special arrangements for cooling; and the depth of each stage in the boring would be restricted to half a mile in order to avoid great cost in the hauling arrangements, great weight of rope, and the great cost of keeping the machinery and workings cool. At each second or third mile down there would be air-locks to prevent the air-pressure from becoming excessive, owing to the weight of the superincumbent air. For when we got between two and three miles down below the surface of the earth the atmospheric pressure there would be double what it is at the earth's surface, or, therefore, about thirty pounds to the square inch. It would not be easy to work under greater air-pressure than that, firstly because of the strain on the workmen, and secondly because of the rise of temperature which this increased air-pressure would cause. Therefore special chambers would have to be constructed to relieve the pressure, as well as special pumps to provide ventilation, and other machinery to carry the superfluous heat to the surface. This last-named machinery would be of the nature of brine-filled pipes, in which a freezing mixture would always be kept circulating. (The arrangements suggested by Mr. Parsons for keeping the underground workings cool are rather too complicated for description here; but no doubt the means he suggests would be effective, and it would be possible, though with great difficulty, to keep the workings cool.)