Before proceeding to introduce the chief actors and their achievements in the period next before us, it will be instructive to glance at some of the principal ideas and methods in favour with aeronauts up to the date now reached. It will be seen that Wise in America, contrary to the practice of Green in our own country, had a strong attachment to the antique mode of inflation with hydrogen prepared by the vitriolic process; and his balloons were specially made and varnished for the use of this gas. The advantage which he thus bought at the expense of much trouble and the providing of cumbersome equipment was obvious enough, and may be well expressed by a formula which holds good to-day, namely, that whereas 1,000 cubic feet of hydrogen is capable of lifting 7 lbs., the same quantity of coal gas of ordinary quality will raise but 35 lbs. The lighter gas came into all Wise's calculations for bolder schemes. Thus, when he discusses the possibility of using a metal balloon, his figures work out as follows: If a balloon of 200 feet diameter were constructed out of copper, weighing one pound to the square foot; if, moreover, some six tons were allowed for the weight of car and fastenings, an available lifting power would remain capable of raising 45 tons to an altitude of two miles. This calculation may appear somewhat startling, yet it is not only substantially correct, but Wise entertained no doubt as to the practicability of such a machine. For its inflation he suggests inserting a muslin balloon filled with air within the copper globe, and then passing hydrogen gas between the muslin and copper surfaces, which would exclude the inner balloon as the copper one filled up. His method of preparing hydrogen was practically that still adopted in the field, and seems in his hands to have been seldom attended with difficulty. With eight common 130-gallon rum puncheons he could reckon on evolving 5,000 cubic feet of gas in an hour, using his elements in the following proportions: water, 560 lbs.; sulphuric acid (sp. g. 1.85), 144 lbs.; iron turnings, 125 lbs. The gas, as given off, was cooled and purified by being passed through a head of water kept cool and containing lime in solution. Contrasted with this, we find it estimated, according to the practice of this time, that a ton of good bituminous coal should yield 10,000 cubic feet of carburetted hydrogen fit for lighting purposes, and a further quantity which, though useless as an illuminant, is still of excellent quality for the aeronaut. It would even seem from a statement of Mr. Monck Mason that the value of coke in his day largely compensated for the cost of producing coal gas, so that in a large number of Green's ascents no charge whatever was made for gas by the companies that supplied him.