218 AIRDRIE AIR PLANTS ually the bubbles become globular or oval, and after the cell walls have increased in thickness, a small canal is formed within the new mass, giving rise to porous vessels. This is readily observable in limes and willows. The air bub- bles obstruct the passage of the sap, and thus cause the consolidation of the wood. The dif- ference between the wood of needle-leafed trees (such as the pine, fir, spruce, larch, &c.) and of broad-leafed trees chiefly depends upon the number of the cells that are converted into porous vessels. AIRDRIE, a borough town of Lanarkshire, Scotland, 11 m. E. of Glasgow; pop. in 1861, 12,922. It is well built, and has recently grown into importance from the extensive coal and iron mines in the neighborhood, and also from its proximity to Glasgow, whence many of its weavers obtain employment. The laird or proprietor of the estate of Airdrie is Mr. A. J. Alexander, a native and resident of Ken- tucky, famous as a breeder of fine horses and cattle in Woodford county. AIRE, a river of Yorkshire, England, rises near Settle, flows S. E., passes Leeds, and re- ceives the Calder at Castleford ; and the two, having been widened and deepened, form one of the links in the canal system of Yorkshire and Lancashire, under the name of the Aire and Calder navigation. From Castleford, the Aire flows E. to the Ouse near Goole. AIRE. I. A fortified city of N. E. France, department of Pas-de-Calais, on the Lys, 8 m. S. S. E. of St. Omer; pop. in 1866, 8,803. It is well built, and has a highly ornamented church. There are manufactures of linen, hats, soap, Dutch tiles, &c. The surrounding district is one of the richest in French Flan- ders. II. A city of S. W. France, department of Landes, on the A^our, 80 m. S. by E. of Bordeaux; pop. in 1866, 4,885. It is a very old place, was formerly strongly fortified, and has suffered much in foreign and civil wars. It was the residence of Alaric II., and has been a bishop's see since the 5th century. AIR GUN, a pneumatic engine resembling a musket, for the purpose of discharging bullets by means of compressed air. It consists of a lock, stock, barrel, and ramrod. The stock is made hollow, and provided with proper cocks for filling it with compressed air by means of a force pump. The lock is nothing but a valve which lets into the barrel a portion of the air compressed in the stock, when the trigger is pulled. The gun is loaded with wadding and ball in the ordinary way, and the air suddenly introduced from the stock propels it with a velocity proportional to the square root of the degree of compression of the air. There is no doubt that if the discovery of powder had not been made at an early date, these in- struments would have reached a point of great effectiveness. The section of the air gun given here represents one of the most practical kind. It has the general form of a musket. The stock A is hollow, and strong enough to withstand the required pressure of the air, being some 50 atmospheres, pumped into it by means of the piston E, moving in the bar- rel C D ; this compressed air is confined in the Air Gun. stock by the spring check valve C. The action of the lock I is such that when pulling the trigger the valve C is for an instant lifted from its seat, which causes a small portion of the air to escape ; and as this air is confined under a pressure of some 50 atmospheres or 750 Ibs., the impulse given to the ball at the first discharge is almost as great as that of gunpowder. One charge may fire several balls, but the effect decreases with every firing. Some air guns have, in place of a hollow stock, a separata large hollow metallic ball into which the air is com- pressed, and which is attached to the side of the lock. These are generally very unsafe. Arms analogous to air guns have been con- trived for producing explosion, with nearly or quite the power of an ordinary musket, by the electrical conversion in the barrel of oxygen and hydrogen, or air and street gas, into steam, and other similar means. AIR PLANTS, a term applied to some species of the families of BromeliaceoR (Tillandsia us- neoides, hanging in festoons from the forest trees of tropical America, moss-like, and T. xiphimdeS) perfuming the balconies of houses in Buenos Ayres, &c.), and of orchidacea (namely, the parasitic groups of them, such as the aerides, arachnides, or flo* aSra of the East Indies, and many others), because of their being able to live for a considerable time, sus- pended in the air, without apparently receiv- ing any nutriment. The hot, damp, and shady forests of the torrid zone in Asia, Africa, and America, abound in gracefully and grotesquely shaped and deliciously scented species of or- chidete, so that in Java alone there are nearly 300 varieties. During the dry season, which is that of repose, corresponding to our winter in this respect, these parasites wither, lose their leaves, and seem to be dead ; but as soon as the gentle, preparatory rain begins to fall, they revive, and become fully developed into their glorious existence by the ceaseless show- ers that transform the whole surface of the country into a magnificent hothouse. They are attached, amid gigantic grasses, ferns, and numberless climbers, to trees, rocks, &c., and are nourished by the continual warm vapors that fill the forests. Stagnant water is injuri- ous to them, even by mere proximity. The roots of most fully developed air plants, by which they cling to their supports high in the air, have an outer parchment-like layer, in which the spiral cells exhibit detached fibres and simple walls ; thus in oncidium altissimum,
