Page:The American Cyclopædia (1879) Volume II.djvu/186

 166 AXMINSTER AXOLOTL to insure a distance between the rims of the wheels invariably equal to that of the rails. It has been attempted to divide axles in the centre, the inner ends of the two half axles being main- tained in boxes fixed in slides on a frame, and the body of the carriage acting as a lever on a small mechanism, and bringing each axle per- pendicular to the curve of the road. One wheel has also been made to revolve around the axle, which was fixed to the other wheel, and turned with it ; in this way railroad cars would turn a short curve without straining the axle. Such arrangements, however, have never been ex- tensively introduced, as the disadvantages from complexity and loss of strength outweigh the advantages gained in turning curves. In horse cars running on city railroads, the difficulty of turning street corners, through curves of very short radius, is simply met by causing the outer wheel to run on its flange on a flat rail ; it thus acts as a larger wheel and passes through a greater distance with the same num- ber of revolutions as the inner wheel, and thus describes a curve, notwithstanding the wheels are all immovably connected with the axles. The difficulty of turning curves is not only in the straining of the axles immovably fixed to the wheels, but also in the rigid parallellism of the forward and rear axles, which opposes the turning of a curve the more in proportion as the car is longer ; and as American passenger cars are very long and curves very common, the so-called truck system was adopted, con- sisting in a frame turning on a vertical axle or pivot, and supported by four or six wheels, of which the axles are parallel. Such a four or six-wheeled frame or truck is placed at each end of the car ; and in going around curves the trucks adapt themselves by turning on the cen- tral vertical pivot. In Europe, where curves are more avoided regardless of expense, and cars are shorter, this system has not been adopted, except in a few exceptional localities, where curves of short radius could not be avoided in the construction of the road. The only kind of locomotive where the wheels are not immovably connected with the axle are those lately built for common roads, in some of which the connection is ingeniously made with a gearing, so that notwithstanding both wheels act as driving wheels, they are not compelled to make the same number of revo- lutions, and thus are able to turn any short curve in a common road. A M I NSTKK, a town in the county of Devon, England, on the left bank of the Axe, 24 m. E. by N. of Exeter ; pop. 2,900. It is well known on account of its rich and beautiful carpets, woven in one piece, which rivalled those of Turkey and Persia ; but the manufacture has now ceased. The town is mentioned in Domes- day Book, and is believed to have existed from very early times. An action was fought near Axminster in the civil wars in 1644. OI.OTI,, the Mexican name of an amphibi- ous reptile, described by naturalists as siredon. This tadpole-formed reptile has the vertebrae biconcave, and the body elongated and formed for swimming. The feet are four, the anterior being four-toed, the posterior five-toed; the sides of the body are marked by several small furrows, and an imperfect lateral line is con- tinued from the gills to the tail. The head is flattened, with a rounded or truncated snout, near the end. of which are the nostrils; the eyes are small, and about midway between the angle of the mouth and the nose ; the tail is elongated and compressed, and tapers to a point. A thin membrane commences near the back of the head, rising gradually to the mid- dle of the tail, and diminishing again toward the tip ; underneath, it extends from behind the vent to the tip, reaching its greatest height at its anterior third. The axolotl belongs to the perennibranchiate order, or those whose gills remain through, life, coexisting with rudi- mentary lungs; hence its respiration is always aquatic. The gill openings are large, and the gill covers are continuous beneath the throat, so as completely to separate the head from tile breast. The gills consist of four semicircular cartilaginous arches, serrated internally like those of fishes, and externally provided with Axoloti. fine branchial fringes, occupying thickly the lower edge of the flaps, and a few on the tip of the upper edge. The fringes are flattened, tapering, and disposed in a double row. A generic character is the presence of four ex- ternal flaps, provided with respiratory fringes. There are two rows of teeth in the upper and lower jaw. There are three species described : siredon Mexicanw, Shaw ; S. maculatw, Ow- en; and S. lichenoides, Baird. It is probable that other species exist, as there are many local- ities in Mexico, New Mexico, and Texas where " fish with legs " are common. The axolotl is about 10 inches long, of a dark brown color, with blackish spots. Great numbers are taken in the month of June from a lake about 3 m. from the city of Mexico, at an elevation of more than 8,000 feet above the level of the sea, and from water whose temperature is never below 60 F. At this time they form the prin- cipal food of the peasantry. From the experi- ments of Prof. O. 0. Marsh, it appears that the axolotl is the larval condition of the salaman- droid batrachian amblystoma, usually regarded