This plan was a great improvement on the hand-pump (p. 70), but it was not quite satisfactory; and it failed when the brook ran dry in summer or was frozen in winter. Also the pump was rather a clumsy one. Papin invented a better instrument, which he called the centrifugal pump, and it is used to the present day. The pump consisted of a round, flat box containing a paddle wheel which whirled the water round. The water entered the box at the centre of the paddle-wheel, and on one side of the box was a pipe through which the water escaped on being whirled round. Papin tried to use this pump for pumping water, but he found that he had no machine for moving it quickly enough, nor did he get such a machine until he had invented the steam-engine. This, however, he did not succeed in doing for many years, and meanwhile he invented a number of other instruments. One of these was a submarine boat. The first man to invent a diving-bell which could move under water was named Drebbel. He was said to have taken a trip under the Thames, disappearing entirely under water and coming up in a distant place. Some years after this, Boyle asked a medical doctor who had married a daughter of Drebbel, how it was that Drebbel and his crew were not suffocated under the water. He was told that Drebbel possessed a certain extract of air which enabled him to breathe in a confined space. We are nowadays not able to tell how much truth there was in the story concerning Drebbel. [Illustration: Fig. 43.--An early form of Diving-Bell.] But it is certain that Papin succeeded in constructing a diving-bell, and in using it under water. His diving-bell was not a bell at all, but a round box 6½ feet high, in which three men could stand upright. When sunk under water it did not disappear entirely, because a pipe stuck out of the top through which fresh air could enter or the used-up air be pumped out. The pump which did this was one of Papin’s own centrifugal pumps. On one side of the diving-bell was a very wide tube, through which a man could crawl. It had a little door at the extreme end which could be opened under water. This would, of course, let in the water, but Papin, with one of his pumps, drove air through the tube from within the diving-bell. This stream of air kept the water back, and the poor man in the tube (he must have been very uncomfortable) could do some work in the water outside, such as laying a mine for exploding an enemy’s ship.