854 TREDGOLD founded by a colony from Sinope, and was a nourishing town when Xenophon arrived there on his retreat from Cunaxa. It became sub- ject to the Romans by conquest from Mith- ridates. The emperor Trajan constructed a mole to improve the port, and made it the capital of eastern or Cappadocian Pontus. During the reign of Gallienus it was plundered and nearly destroyed by the Goths, but in the time of Justinian it had completely recovered, and was made the capital of a province which included Pontus and some part of Armenia. In 1204 it became the seat of an independent branch of the Comnenus family (see ALEXIS, and BYZANTINE EMPIRE), under whom the ter- ritory was called the empire of Trebizond. It continued subject to this line of rulers until it was conquered by the Turks in 1401. TREDGOLD, Thomas, an English engineer, born at Brandon, near Durham, Aug. 22, 1788, died in London, Jan. 28, 1829. In 1808 he went to Scotland, where he worked five years as a journeyman carpenter and joiner, studying ardently in spare hours. In 1813 he went to London, and entered the service of William Atkinson, architect to the ordnance, and in 1823 commenced as civil engineer on his own account. He published " Elementary Princi- ples of Carpentry" (4to, London, 1820); "A Practical Essay on the Strength of Cast Iron and other Metals" (8vo, 1821); "The Princi- ples of Warming and Ventilating Public Build- ings, Dwelling Houses," &c. (2d. ed., 1824); "A Practical Treatise on Railroads and Car- riages" (1825); "Remarks on Steam Naviga- tion, and its Protection, Regulation, and En- couragement" (1825); and "The Steam En- gine, comprising an Account of its Invention and progressive Improvement, with an Inves- tigation of its Principles " (4to, 182V), a later edition of which (1850-'53) is in 4 vols. 4to, with 226 steel plates and 164 woodcuts. TREE FROG, the name of the batrachian reptiles of the family hyladce, distinguished from common frogs (ranidce) by having the ends of the fingers and toes dilated into flat- tened disks or suckers, which enable them to lead an arboreal life. They are more elegant, smaller, brighter, and more active than the ranidce, and are lively during the day; they feed on insects; they climb like the geckos among lizards, and by the same mechanism ; the lower surface of the disks is endued with a viscid secretion, by means of which they can walk with the body suspended from the under parts of leaves and other smooth bodies ; the skin is mostly smooth upon the back, but on the abdomen and inside of legs thickly studded with small warts or tubercles. They possess to a remarkable degree the faculty of chang- ing color, which enables them to elude their numerous enemies. They are very clamorous, and particularly noisy at the approach of rain ; in winter they bury themselves in the mud at the bottom of pools ; they breed in the spring, depositing their eggs in the water. They are TREE FROG frequently called tree toads, and their French name is rainettes. The species are numerous, especially in America; only one is found in Europe, and that also occurs in N. Africa and Asia. The common tree frog of North Amer- ica (hyla versicolor, Le Conte) resembles a toad in form, but is more flattened ; body short and warty above, the color varying from pale ash to dark brown, with several large irregular blotches of greenish brown, white and granu- lated below, and abdomen yellowish near the thighs ; the colors vary at the will of the ani- mal. The head is short and rounded, the mouth large, with teeth on upper jaw and vomer ; eyes large and brilliant, the iris bright golden ; there are four fingers and five toes, both end- ing in viscous pellets, the former distinct, but the latter webbed for four fifths of their length. It is about 2 in. long, and is found abundantly in the northern and middle states, and as far west as the Mississippi ; it is generally seen on Common Tree Frog (Hyla versicolor). decaying trees and about old fences of wood or stone, overgrown with mosses and lichens, the color of which it so nearly resembles that it is very difficult to detect ; it is very noisy in spring and summer toward evening, especially in cloudy weather ; the secretion of the skin is copious and very acrid. This species is re- placed in the southern states by the green tree frog (H. viridis, Laur. ; calamita Carolinensis, Penn.), which is bright green above, yellowish white below, with a straw-colored lateral line extending from the upper jaw over the shoulder and along the side. The tree frog of Europe (H. arborea, Linn.) much resembles the green species of North America, and the latter was considered by Laurenti a variety of his H. riri- dis; it is spread throughout Europe, except in Great Britain. In the jienus acris (Dum. and Bibr.) the locomotive disks are less developed and the limbs more slender than in hyla, and there are teeth on the palate instead of the
