Nearly 3,000 United States patents have been granted in the class of spinning, and many valuable improvements in the details of construction in spinning machinery have been made in recent years. The most important, perhaps, are those relating to spindle structure, whereby the speed and efficiency of spinning machines have been greatly increased. Prior to 1878 the speed of the average spindle was limited to 5,000 revolutions a minute. In 1878 improvements were made which doubled its working speed and permitted as high as 20,000 revolutions a minute. This result was accomplished by making a yielding bolster. The bolster is an upright sleeve bearing, in which the spindle revolves, and against which is sustained the pull of the band that drives the spindle. By making this bolster or sleeve bearing to yield laterally by means of an elastic packing which surrounds it, a much greater freedom and speed of revolution were obtained. The preliminary step in this direction was made by Birkenhead in patent No. 205,718, July 9, 1878. In the same year this idea was perfected by Rabbeth. The bolster was placed loosely in a bolster case of slightly larger diameter than the bolster, and the bottom of the spindle had a free lateral movement as well as the top, as shown in his patent No. 227,129, May 4, 1880. With such perfect freedom of movement, the spindle at high speed could find its own center of revolution, and an indefinitely high speed and quadrupled efficiency were attained. The Draper Spindle is shown in Fig. 289 as one of the most modern and representative of spinning spindles. Considering the great speed of the modern spindle and the fact that a single workman attends a thousand or more of them, the record of progress in this art becomes impressive. In 1805 there were only 4,500 cotton spindles at work in the United States. In 1899 there were 18,100,000.