4. _The Mule._ In 1779, Crompton invented a mule, by which threads of a finer and stronger quality could be spun, and thus made it possible to weave any grade of cloth. 5. _The Power-loom._ The spinners were now able to keep ahead of the weavers, till Cartwright invented, in 1785, a power-loom that enabled the weavers to work faster and use all the thread that the spinners could make. 6. _The Steam-engine._ These machines were run by hand or water-power. In 1785, Watts' steam-engine, invented several years before this, was used in the manufacture of cotton, and manufacturers were now able to use all the raw material they could get. The use of steam instead of water-power led to the building of factories in cities, where labour was plentiful and transportation facilities good. This meant large cities. 7. _The Cotton-gin._ Cotton had to be cleaned of its seeds before it could be used in the factory. This had to be done by hand, which greatly hindered the supply of raw material. A good deal of the raw cotton came from the United States, and the planters there grew no more than could be cleaned and sold. In 1792, Eli Whitney, an American, invented the cotton-gin, by which the cotton could be cleaned of its seed very quickly. Formerly a workman could clean by hand only five pounds of cotton a week; by the saw-gin five hundred pounds could be cleaned in an hour. (If a cotton-boll can be procured, the pupils will soon discover how difficult it is to separate the seeds from the cotton.) More cotton was then grown, because it could be sold to the factories, and England was able to get all she required to keep the factories going. It may be added here that the increase in cotton growing required more hands for its cultivation; at that time, this meant more slaves; the cotton-gin was therefore a large factor in the slave troubles in the Southern States that led to the Civil War. 8. _Coal-mining and Smelting._ These machines were made of iron, and coal was needed to run the engines and to smelt the iron. There was plenty of coal in England, but very little was mined until the steam pump was brought into use to keep the mines clear of water. When this was done, more men went to work in the mines to get out the greater amount of coal that was now needed. There was also plenty of iron ore in England, and before this it had been smelted by means of charcoal, which is made from wood. This slow and wasteful method was followed until Roebuck invented a process of smelting by coal, and thus made possible a plentiful supply of iron for the manufacture of the machines. 9. _The Safety Lamp._ Coal-mining was a dangerous occupation, because of the fire-damp that is generated in mines. The open lamps used by the miners often caused this gas to explode and many men lost their lives thereby. To remedy this, Sir Humphrey Davy invented the safety lamp in 1815, which gave the miners the light they needed and prevented these explosions.