Taking advantage of what had been taught them by Watt and others as to steam and steam engines, and of the principles and laws of gases as expounded by Boyle, Mariotte, Dalton, and Gay-Lussac, that many of the gases, such as air, preserve a permanent expansive gaseous form under all degrees of temperature and compression to which they had as yet been subjected, that when compressed and released they will expand, and exert a pressure in the contrary direction until the gas and outside atmospheric pressure are in equilibrium, that this compressed gas pressure is equal, and transmitted equally in all directions, and that the weight of a column of air resting on every horizontal square inch at the sea level is very nearly 14.6 pounds, the inventors of the nineteenth century were enabled by this supreme illumination to enter with confidence into that work of mechanical contrivances which has rendered the age so marvellous. It was natural that in the first development of mechanical appliances they should be devoted to those pursuits in which men had the greatest practical interest. Thus as to steam it was first applied to the raising of water from mines and then to road vehicles. And so in 1800 Thos. Parkinson of England invented and patented an “hydrostatic engine or machine for the purpose of drawing beer or any other liquid out of a cellar or vault in a public house, which is likewise intended to be applied for raising water out of mines, ships or wells.” By the use of a sort of an air pump he maintained an air pressure on the beer in an air-tight cask situated in the cellar, which was connected with pipes having air-tight valves, with the upper floor. The liquid was forced from the cellar by the air pressure, and when turned off, the air pressure was resumed in the cask, which “preserved the beer from being thrown into a state of flatness.” Substantially the same device in principle has been reinvented and incorporated in patents numerous times since. In the innumerable applications of the pneumatic machines and air tools of the century, especially of air-compressing devices, to the daily uses of life, we may, by turning first to our home, find its inner and outer walls painted by a pneumatic paint-spraying machine, for such have been made that will coat forty-six thousand square feet of surface in six hours; and it is said that paint can be thus applied not only more quickly, but more thoroughly and durably than by the old process. The periodical and fascinating practice of house cleaning is now greatly facilitated by an air brush having a pipe with a thin wide end in which are numerous perforations, and through which the air is forced by a little pump, and with which apparatus a far more efficient cleaning effect upon carpets, mattresses, curtains, clothes, and furniture can be obtained than by the time-honoured broom and duster.