fathoms the bell must be held fast, in order to expel the water which has rushed in, by letting fresh air into it. By such apparatus, Halley was enabled to make the bottom of the sea, within the circumference of the bell, so dry that the sand or mud did not rise above his shoe. Through the window, in the top, so much light was admitted, that when the sea was still and the waves did not roll, he could see perfectly well to read and write under the water. When the empty air-vessels were drawn up, he sent up with them his orders, written with an iron spike on a plate of lead, and could thus let those above know when he wished to be removed with the bell to another place. In bad weather, and when the sea was rough, it was as dark under the bell as at night; he then kindled a light; but a burning candle consumed as much air as a man. The only inconvenience of which Halley complained was, that, in going down, he felt a pain in his ears, as if a sharp quill had been thrust into them. This pain returned every time the bell was let down to a greater depth, but soon went off again. A diver thought to prevent this pain by putting chewed paper into both his ears; but the bits of paper were forced in so far by the air, that a surgeon found great difficulty to extract them. Another improvement of the diving-bell was effected by the well-known Triewald, a Swede, in 1732. His bell, which was much smaller and more commodious, was made of copper, tinned in the inside. On the top there were panes of glass, which, for the greater security, were fixed in a frame of the same metal. The stool below was placed in such a manner, that the head only of the diver, when he stood upon it, rose above the surface of the water in the bell. This situation is much better than when the whole body is raised above the water in the bell, because near the surface of the water the air is much cooler and fitter to breathe in than at the top of the machine. That the diver also might remain conveniently in the upper part of the bell, Triewald arranged his apparatus so that when the diver had breathed as long as possible in the upper air, he found at the side of the bell a spiral pipe, through which he could draw in the lower cool air which was over the surface of the water. To the upper end of this copper pipe was affixed a pliable leather one, with an ivory mouth-piece, which the diver put into his mouth, and could thus inspire fresh air, in whatever position his body might be[298].