It was found that by making apertures in the vessel's sides at about three feet from the water and dividing the space between the (zyga) thwarts, room could be made for a second row of men with shorter oars, but still handy and able to add to the propulsion of the vessel. For these seats were found in the hold (thalamus), and hence while the upper tier of the bireme took their name from the zyga, benches or thwarts, and were called 'Zygites,' the men of the lower tier were called 'Thalamites.' These names were continued when the invention of the 'thranos,' or upper seat, had added a third or upper tier with longer oars to the system, and so introduced the trireme. If the number of the zygites in the penteconter was twenty-five a side, and the first bireme was a converted vessel of that class, the number of thalamites, owing to the contraction of the bow and the stern, would necessarily be two or three a side less. Thus we may consider a converted penteconter to have been capable of carrying a rowing crew of between 90 and 100 men. Similarly a triaconter would have been capable of adding nearly twenty pairs of arms to her propelling power. When, in consequence of the new invention, vessels were expressly built as triremes, we may imagine that for convenience' sake the benches or zyga would be a little raised, so as to give more room for the raised seat of the thalamites that was fastened on to the floor of the vessel. The narrowness of the vessels affected the disposition of the rowers in the Greek galleys in a peculiar way. It is evident from the testimony of the ancients that they adhered strictly to the principle of 'one man to each oar.' The arrangement seen in mediaeval galleys was absolutely unknown to them, and would not have suited them. It belongs to a different epoch and a different order of things, when the invention of the 'apostis' had made the use of large sweeps rowed by two or three men possible, and a vessel with sets of three rowing upon the same horizontal plane might be called a trireme, though utterly unlike the ancient vessel of that name.