The remaining method of producing rotation by means of a string fixed to the shaft can be seen in use by boatmen when clearing water out of a boat with a mop, The mop is laid over the side of the boat. A few turns of a rope fixed to the shaft are made round it and the rope being pulled the shaft rotates. The momentum generated causes the shaft to overrun and the rope to coil itself in the reverse way to the original. This is repeated till the speed generated causes the water to fly off the mop by centrifugal motion. The fire drill of the ancient Egyptians was turned by a bow, and it is interesting in connexion with the advice of Hippocrates to avoid generating too much heat in drilling the skull, and also because it helps to explain the construction of the instruments of Vidius. A sketch of an ancient fire drill found by Flinders Petrie (_Ten Years Digging in Egypt_) shows that the head of the drill was separate and the points were also removable. _Drill with Guard._ Greek, τρύπανον ἀβάπτιστον; Latin, _terebra abaptista_. This is the second variety of drills described by Celsus. It had a collar which prevented it from sinking beyond a certain depth, so that in excising a piece of bone from the skull, which was the object for which it was used, there was little danger of its doing injury to the brain or its membranes: Terebrarum autem duo genera sunt; alterum simile ei quo fabri utuntur; alterum capituli longioris, quod ab acuto mucrone incipit, deinde subito latius fit; atque iterum ab alio principio paulo minus quam aequaliter sursum procedit (VIII. iii). Further on in the same passage Celsus states that they were to be frequently removed and dipped in water lest too great heat should be generated, so that they were evidently driven at a rapid rate with a thong like the other drills. They are not mentioned by Hippocrates, but Galen (x. 445) describes them: 'In order to make less chance of error they have invented drills called abaptista (ἀβάπτιστα τρύπανα), which have a circular border a little above the sharp point of the drill. It is best to have several for every thickness of the calvarium; for thicker bone longer are required, for thinner bone shorter.' Paul (VI. xc) says: 'But if the bone is strong it is first to be perforated with that kind of perforators called abaptista (περιτρυπήσαντες ἀβαπτίστοις τοῖς λεγομένοις), which have certain eminences to prevent them sinking down to the membrane, and then with chisels we remove the bone not whole, but in pieces.' The illustrations of drills given from Vidius (Pl. XLII) are really abaptista. _Saw._ Greek, πρίων, μαχαιρωτὸς πρίων (as if from μαχαιρόω); Latin, _serrula_. The saw is very frequently mentioned in the description of operation on bone. Celsus (VII. xxxiii), in describing the amputation of a gangrenous limb, says: