Is the home uncomfortable by reason of heat and summer insects? A compressor having tanks or cisterns in the cellar filled with cool or cold air may be set to work to reduce the temperature of the house and fan the inmates with a refreshing breeze. Air engines have been invented which can be used to either heat or cool the air, or do one or the other automatically. The heating when wanted is by fuel in a furnace forced up by a working cylinder, and the cooling by the circulation of water around small, thin copper tubes through which the air passes to the cylinder. Do the chimes of the distant church bells lead one to the house of worship? The worshipper goes with the comforting assurance that the chimes which send forth such sweet harmonies are operated not by toiling, sweating men at ropes, but by a musician who plays as upon an organ, and works the keys, valves and stops by the aid of compressed air, and sometimes by the additional help of electricity. Mention has already been made of office and other elevators, in which compressed air is an important factor in operating the same and for preventing accidents. If a waterfall is convenient, air is compressed by the body of descending water, and used to ventilate tunnels, and deep shafts and mines, or drive the drills or other tools. The pneumatic mail tube despatch system, by which letters, parcels, etc., are sent from place to place by the force of atmospheric pressure in an air-exhausted tube, is a decidedly modern invention, unknown in use even by those who are still children. Tubes as large as eight inches in diameter are now in use in which cartridge boxes are placed, each holding six hundred or more letters, and when the air is exhausted the cartridge is forced through the tubes to the distance sometimes of three miles and more in a few minutes. In travelling by rail the train is now guided in starting or in stopping on to the right track, which may be one out of forty or fifty, by a pneumatic switch, the switches for the whole number of tracks being under the control of a single operator. The fast-moving train is stopped by an air brake, and the locomotive bell is rung by touching an air cylinder. The “baggage smashing,” a custom more honoured in the breach than in the observance, is prevented by a pneumatic baggage arrangement consisting of an air-containing cylinder, and an arm on which to place the baggage, and which arm is then quickly raised by the cylinder piston and is automatically swung around by a cam action carrying the baggage out of or into the car. Bridge building has been so facilitated by the use of pneumatic machines for raising heavy loads of stone and iron, and for riveting and hammering, and other air tools, aided by the development in the art of quick transportation, that a firm of bridge builders in America can build a splendid bridge in Africa within a hundred days after the contract has been entered upon.