OF GALILEO.

holds them would be much more difficult than to break them? But in the rope, the very act of twisting it squeezes the threads mutually among themselves, in such a manner that by then pulling the rope with great force, its filaments break, and do not separate one from the other; as is manifestly known from seeing in the break the threads very clearly, and not at least one braccia long each, as should be seen if the division of the rope were made not by the tearing of the threads, but by the simple separation of one from the other by sliding.

Sagr. Let us add in confirmation of this the observation that sometimes a rope breaks not by pulling it lengthwise, but only by excessively twisting it: an argument that seems to me conclusive that the threads are so mutually compressed among themselves, that the compressing ones do not permit the compressed ones to slide that minimum which would be necessary to lengthen the coils so that they could encircle the rope, which in twisting shortens itself, and in consequence thickens somewhat.

Salu. You speak very well: but consider next how one truth pulls another behind it. That thread which, held between the fingers, does not follow the one who would like to withdraw it from between them by pulling it with some force, resists because it is retained by a double compression, even though the upper finger presses no less against the lower than this one presses against that. And there is no doubt that if one could retain only one of these two pressures, there would remain half of that resistance which depended on the two joined: but because one cannot, by raising, for example, the upper finger, remove its pressure without also removing the other part, it is necessary with a new artifice to conserve one of them, and find a way that the same thread compresses itself against the finger, or other solid body upon which it rests, and make it so that the same force that pulls it to separate it from it, compresses it all the more against it, the more forcefully it pulls: and this will be achieved by wrapping the same thread like a spiral around the solid. In order that this be better understood, I will draw a little figure; and let these AB, CD be two cylinders, and between them stretched the thread EF, which for