It is certain that the air-chamber, at least in Germany, came into common use after it was applied by Leupold to fire-engines, a great number of which he manufactured and sold. He gave an account of it in a small work, consisting of four sheets quarto, which was published in 1720, but at first he kept the construction a secret. The engines which he sold consisted of a strong copper box closely shut and well-soldered. They weighed no more than sixteen pounds, occupied little room, had only one cylinder; and a man with one of them could force up the water without interruption to the height of from twenty to thirty feet. About 1725 Du Fay saw one of Leupold’s engines at Strasburg, and discovered by conjecture the construction of it, which he made known in the Transactions of the Academy of Sciences at Paris for that year. It is very singular that on this occasion Du Fay says nothing of Mariotte, or of the engine in the king’s library. Leupold, however, had some time before, that is in 1724, given a description and figure in his Theatrum Machinarum Hydraulicarum[608], with which undoubtedly Du Fay was not acquainted. Another improvement, no less useful, is the leather hose added to the engine, which can be lengthened or shortened as necessary, and to which the fire-pipe is applied, so that the person who directs the jet of water can approach the fire with less danger. This invention, it is well known, belongs to two Dutchmen, both named Jan van der Heide[609], who were inspectors of the apparatus for extinguishing fires at Amsterdam. The first public experiments made with it took place in 1672; and were attended with so much success, that at a fire next year, the old engines were used for the last time, and the new ones introduced in their stead. In 1677, the inventor obtained an exclusive privilege to make these engines during the period of twenty-five years. In 1682, engines on this construction were distributed in sufficient number throughout the whole city, and the old ones were entirely laid aside. In 1695 there were in Amsterdam sixty of these engines, the nearest six of which were to be employed at every fire. In the course of a few years they were common throughout all the towns in the Netherlands.