In the year following the invention of gun-cotton, came Sobrero's invention of nitro-glycerin, made by the action of nitric acid on glycerin (1847). The new explosive was more powerful than gun-cotton, but much more dangerous to handle. By reason of its extreme sensitiveness and the consequent danger of handling it, the use of pure nitro-glycerin has never been great. In the same year, 1847, the time-lock was invented by Savage. This invention was in the class with the gun and gun-cotton, in the sense that it enhanced the self-protectiveness of the Machine. It did not enhance its self-protectiveness against a few great, open, external foes, however, but against a myriad of small, secret, internal foes. The Machine is very expensive to maintain in operation, and so is every one of the little mechanisms of which it is composed. And each one of these little mechanisms, each bank, its business corporation, each company, each department store, each little shop, requires that its money be kept safe from the burglar and the pilferer. Inasmuch as the time-lock assists in doing this, the time-lock has been a valuable contribution to the Machine, and has exerted a good influence on history since it was invented. In the same year, 1847, R. M. Hoe invented his great printing press, that could make 20,000 impressions per hour. As it was a long step forward in the improvement of printing, this invention deserved the applause which it received; and the inventor deserved the financial reward which he received. In 1848, Dennison invented a machine for making matches. This was a most useful contribution; but one is inclined to wonder why twenty years elapsed between the invention of matches and the invention of a machine for making them. Inventing was not going ahead so fast then as it is now. Surely, no such interval is allowed to pass unutilized, in the present inventing days.