Archimedes knew, of course, that the matter was not quite so simple as that. He knew that the lever required to move the earth would have to be exceedingly stout and strong, as thick, indeed, as the earth itself. For, otherwise, it would not stand the strain. Besides, he knew he would also require a fixed support against which to rest the lever. And the place on which he was to stand would have also to be firmly fixed, so as to allow him to exert the force required. We know now that the earth floats freely in space, and that there is no fixed place anywhere. Therefore we cannot, after all these 2200 years, try the great experiment suggested by Archimedes. But many other things which Archimedes proposed are now used every day. One of these is the Endless Screw, by means of which Archimedes is said to have drawn a ship, with cargo and crew, along the sand as easily as if it were floating in water, to the great delight of his friend the king. Not long after this the king showed Archimedes a great mark of favour and confidence. This is how it came about. [Illustration: Fig. 3.--Archimedes’ Screw can be used to raise Water.] The king had given one of his goldsmiths a certain weight of gold, and instructed him to make it into a golden crown which he wanted to present to one of the temples. After a few weeks the goldsmith came to the palace and brought the crown to the king. The king weighed the crown, and found it was the same weight as the gold he had given out. But somebody told him that the goldsmith had mixed a quantity of silver with the gold, and kept the remaining gold for himself. The king was a just man, and did not want to accuse the goldsmith of the crime without having a proof of his guilt. He therefore sent for Archimedes and gave him the crown, asking him to find out whether any silver had been mixed with the gold, or whether it was made of pure gold. Archimedes was long puzzled over this problem. He weighed the crown, and found the weight was correct. It also looked like pure gold, so that the amount of silver could not be very much. Archimedes made two blocks, one of silver and the other of gold, of exactly the same size, and found that the block of gold weighed nearly twice as much as the block of silver. “Now,” he said to himself, “if I could melt up the crown and make it into a square block, and make another block of exactly the same size of pure gold, both should weigh the same if the crown is pure gold, but if it is not, the block made from the crown should be lighter.” He wished for a moment to melt up the crown, but it was beautifully made, and he thought it a pity to throw away the fine work. If he could only find out the exact bulk of the crown, he thought he could determine easily if it was the proper weight which it should be if made of pure gold. The problem then was how to find out the total bulk of the crown without melting it up into a block.