63. The slowness with which powders subside, depends partly on the specific gravity of the substance, and partly on the magnitude of the particles themselves. Bodies, in falling through a resisting medium, after a certain time acquire a uniform velocity, which is called their terminal velocity, with which they continue to descend: when the particles are very small, and the medium dense, as water, this terminal velocity is soon arrived at. Some of the finer powders even of emery require several hours to subside through a few feet of water, and the mud pumped up into our cisterns by some of the water companies is suspended during a still longer time. These facts furnish us with some idea of the great extent over which deposits of river mud may be spread; for if the mud of any river whose waters enter the Gulf Stream, sink through one foot in an hour, it might be carried by that stream 1,500 miles before it had sunk to the depth of 600 or 700 feet. 64. A number of small filaments of cotton project from even the best spun thread, and when this thread is woven into muslin they injure its appearance. To cut these off separately is quite impossible, but they are easily removed by passing the muslin rapidly over a cylinder of iron kept at a dull red heat: the time during which each portion of the muslin is in contact with the red-hot iron is too short to heat it to the burning point; but the filaments being much finer, and being pressed close to the hot metal, are burnt. The removal of these filaments from patent net is still more necessary for its perfection. The net is passed at a moderate velocity through a flame of gas issuing from a very long and narrow slit. Immediately above the flame a long funnel is fixed, which is connected with a large air-pump worked by a steam-engine. The flame is thus drawn forcibly through the net, and all the filaments on both sides of it are burned off at one operation. Previously to this application of the air-pump, the net acting in the same way, although not to the same extent, as the wire-gauze in Davy's safety lamp, cooled down the flame so as to prevent the combustion of the filaments on the upper side: the air-pump by quickening the current of ignited gas, removes this inconvenience. NOTES: 1. The importance and diversified applications of the steam engine were most ably enforced in the speeches made at a public meeting held (June 1824) for the purpose of proposing the erection of a monument to the memory of James Watt; these were subsequently printed. 2. Some observations on the subject, by Dr Fitton, occur in the appendix to Captain King's Survey of the Coast of Australia, vol. ii, p. 397. London, 1826. Chapter 8 Registering Operations