297 



TRACTION. 



TRADE, BOARD OF. 



circumstances of each case : if the obstruction is removed, the wound 

 in the trachea may be healed, but if not, the canula must be worn for 

 the rest of life. If the operation were performed for the removal of a 

 foreign body, or for the inflation of the lungs, no canula need be intro- 

 duced, but the wound should be closed as soon as the main object is 

 attained. 



TRACTION', in Mechanics, is the act of drawing a body along a 

 plane, usually by the power of men, animals, or steam; as when a 

 vessel is towed on the surface of water or a carriage moved upon a 

 road. The power exerted in order to produce the effect is called the 

 force of traction. 



Numerous experiments have been made for the purpose of ascer- 

 taining the value of a force so exerted ; and when men are employed 

 to draw laden boats on canals, it is found that if the work be continued 

 for several days successively, of eight hours each, the force of traction 

 is equivalent to a weight of 314 Ibs., moved at the rate of two feet per 

 second, or 1J mile per hour (it being understood that such weight is 

 imagined to be raised vertically by means of a rope passing over a 

 pulley, and drawn in a horizontal direction). The force of traction 

 exerted when, without moving from his place, a man pulls horizontally 

 against a weight so suspended, is estimated at 70 Ibs. The action of a 

 horse in drawing a vessel on a canal is said to be equivalent to a 

 weight of 180 Ibe. raised vertically, as above supposed, with a velocity 

 of 3J feet per second, or 2J miles per hour ; but this estimate has been 

 considered too high ; and from experiments which have been made on 

 the power of horses in waggons, carta, and coaches, on level ground, it 

 is found that the force of traction exerted by a stout horse is equivalent 

 to 80 Ibs. raised at the rate of 4j feet per second, or 3 miles per hour. 

 Mr. Tredgold oonsiders that a horse exerts a force of traction expressed 

 by 125 Ibs. raised at the rate of 3 feet per second, or 2$ miles per hour. 

 A man or a horse can however double his power of traction for a few 

 minutes without being injured by the exertion ; and when the carriage 

 is in motion, so that the friction on the ground is alone to be over- 

 come, a horse can draw, during a short time, on a level road, a weight 

 exceeding 1500 Ibs. 



The force of traction is found to vary nearly with the term (w r) s , 

 where w is the greatest walking velocity of a man or horse when 

 unresisted (6 feet per second, or 4 miles per hour, for a man ; and 1 

 feet per second, or 6 miles per hour, for a horse), and v is the velocity 

 with which the vessel or carriage is moved. From theoretical con- 

 siderations it has been determined that the greatest effect is produced 

 when the velocity of the object moved is one-third of that with which 

 the man or animal can walk when unresiated. 



If a wheel-carriage were situated on a level plane which opposed no 

 resistance, it is evident that, whatever were the diameter of the wheels, 

 the smallest conceivable power of traction applied to the axle would 

 suffice to put the carriage in motion. But when a wheel in moving 

 meets with an obstacle on the ground, that obstacle in pressed at the 

 point of contact by a force acting in the direction of a line drawn to it 

 from tin: centre of the wheel, and arising from that part of the weight 

 which is supported by the wheel, together with the force of traction ; 

 therefore by the " resolution of forces," the ratio between the resistance 

 which is to be overcome by the moving-power and the weight on the 

 wheel will become less as the diameter of the wheel is increased : also 

 the most advantageous direction in which the force of traction can be 

 exerted is perpendicular to the line of pressure drawn from the centre 

 of the wheel to the obstacle. But the height of the wheels cannot 

 exceed certain limits depending on the use to which the carriage is 

 applied ; and when the latter has four wheels, the height of those 

 which are in front must be such only as will allow it to be turned 

 round within a given space; also, when a horse is employed to move a 

 _;>, aUrntiou must be paid to the conditions under which his 

 i may be advantageously exerted. 



It was first observed by M. Deparcieux, and published in the 

 ' Mrinoires de 1'Acaddmie des Sciences,' 1760, that horses draw heavy 

 loads rather by their weight than by their muscular force. Dr. (Sir 

 I ) Brewster has also remarked that when the resistance is great a 

 horse lifts both its fore- feet from the ground ; then, using his hinder- 

 fei't as a fulcrum, he allows his body to descend by its weight, and 

 thus overcomes the obstacle : and it may be added that when this 

 action takes place with a two-wheeled carriage, if the loading is dis- 

 posed so that some portion of it may press on the horse's back, the 

 effect of the animal's weight will thereby be increased. Now if the 

 traces, or the shafts of the carriage, were attached to the horse's collar 

 near his centre of gravity, a line imagined to be drawn from the latter 

 point to his hinder-feet may represent his weight, and a line drawn 

 perpendicularly from his feet upon a plane passing through the traces 

 or shafts may represent the lever of resistance : but while the former 

 line remains the same, this lever becomes less as the plane of traction 

 (that of the traces or shafts) inclines more upwards from the wheel ; 

 and therefore, in order that the power of the horse may be advan- 

 tageously applied, the diameter of the wheel should be as small as is 

 -istcnt with other circumstances. 



Experiments have shown that when the angle of traction, as it is 



called, that is, the angle which the plane of the traces makes with the 



11 which the carriage is moving, is 16 or 16 degrees, a horse pulls 



with good effect . and the height of the points at which the traces are 



attached to a horse's collar being about 4 feet inches from the 



ground, it follows that, in order to obtain this inclination, the lower 

 extremities of the traces or shafts should be 2 feet 3 inches from the 

 ground. In general however, in two-wheeled carriages, the height of 

 these extremities is about 3 feet. 



As an example of the force of traction exerted by steam, it may be 

 stated that on a level line of railway, an engine with an 11 -inch 

 cylinder, and having an effective pressure of 50 Ibs. per square inch in 

 the boiler, drew 50 tons at the rate of 30 miles per hour, working 10 

 hours daily ; and that the same engine, with an equal pressure in the 

 boiler, drew 160 tons at the rate of 15J miles per hour. (Pambour 

 ' On Locomotive Engines.') The resistances to be overcome, or, in 

 other words, the efforts of traction required upon the various systems 

 of intercommunication, may be stated as follows : 



Ratio of Traction 



to weight. 

 Ordinary macadamised roads in good state, horse walk- ) j to 20 or 25 



ing at useful Telocity ..... ) 

 Ordinary macadamised roads in good state, horse trotting 1 to 14 



Ordinary macadamued roads, paved, horse walking 

 Wooden road ,, 



Tramway, of granite blocks, horse walking . 

 Railways, Telocity 20 miles per hour ... 



,, 30 miles per honr . . . 

 Canal of small section, small velocity, 2 iriles per hour 

 ,* , 4 miles 



8 miles 



Canal of large section, 2 miles per hour . . . 

 4 miles per hour . . 



8 miles per hour ... 



1 to SO 

 1 to 70 

 1 to 180 

 1 to 200 

 to 100 

 to GOO 

 to 150 

 to 37 

 to 1000 

 to 250 

 to 62 



TRACTRIX, or TRACTORY, the name given to a curve described 

 by a heavy point attached to a string, the other end of which is 

 moved along a given straight line or curve. For some account of this 

 curve, which is of no interest except as a mathematical exercise, see 

 Peacock's Examples, page 174. 



TRADE, BOARD OF. The department of the English government 

 popularly known under this title is a committee of the Privy Council, 

 and its proper designation, which correctly defines its principal 

 functions, is " The Lords of the Committee appointed for the con- 

 sideration of all matters relating to Trade and Foreign Plantations." 

 This department is practically under the direction of a president and 

 vice-president ; the other members of the Board or Committee are, 

 the Lord Chancellor, the Archbishop of Canterbury, the First Lord 

 of the Treasury, the principal Secretaries of State, the Chancellor 

 of the Exchequer, the Speaker of the House of Commons, the 

 Chancellor of the Duchy of Lancaster, the Paymaster of the Forces, the 

 Treasurer of the Navy, the Master of the Mint, and such officers of 

 state in Ireland as are privy councillors in England ; but those 

 functionaries do not ordinarily interfere with or assist at the delibera- 

 tions of the president and vice-president. The clerks of the council 

 are, ex officio, secretaries of the Board of Trade, but that duty is 

 performed by two joint assistant-secretaries. 



The president and vice president of the Board of Trade exercise, in 

 effect, the duties which in other countries are performed by the 

 minister of commerce. Their office is not indeed executive, but rather 

 consultative, the orders rendered necessary by their decisions being 

 given by the Lords of the Treasury or by the secretary of state, as the 

 case may require. The functions of this board have been of late years 

 considerably extended, its duties being some of them of a ministerial, 

 and others of a judicial character. It has the general superintendence of 

 mutters relating to merchant ships and seamen, and the carrying into 

 execution of the statutes in force relating to them. For that purpose 

 it has to require and receive various kinds of returns as to trade and 

 navigation, and originate and consider reports made to it by its 

 inspectors and other officers. It has also a partial control over local 

 marine boards, and may lay down rules as to the conduct of examina- 

 tions, and as to the qualification of applicants for the posts of masters 

 and mates of foreign-going as well as of home-trade passenger-ships. 

 [SHIPS.] It grants licences to persons to engage or supply seamen or 

 apprentices for merchant ships in the United Kingdom, adjudicates on 

 claims for wages, and investigates cases of alleged incompetency and 

 misconduct (17 4 Is Viet. c. 104). The Board also appoints officers to 

 report on the condition of steam-vessels and their machinery (14 & 15 

 Viet. c. 79). 



The Board of Trade exercises a supervision over railways and railway 

 companies, not only with respect to their original formation, but also 

 as to their subsequent working. Railways were first placed under this 

 control by the statute 344 Viet. c. 97. A few years afterwards the 

 powers of the Board in this respect were transferred to a Board of 

 Commissioners of Railways; but in 1851 all the powers of this latter 

 board were transferred to the Board of Trade (14 & 15 Viet. c. 64). 

 Notices of application for Railway Acts, accompanied by plans, must 

 be deposited with the Board, before any bill can be introduced into 

 Parliament; and before a line can be opened for traffic, notice 

 must be given to the Board, and its permission obtained, on the 

 report of an inspector, appointed by the Board for those and other 

 general purposes. So, when accidents occur, notice must be given to 

 the Board, and an inspector is generally sent to inquire into the 

 circumstances, and on his report the Board may cause alterations to 

 be made for the greater safety of the public. 



