At that time (1816) the rails were made of cast iron, each rail being about three feet long; and sufficient care was not taken to maintain the points of junction on the same level. The chairs, or cast-iron pedestals into which the rails were inserted, were flat at the bottom, so that whenever any disturbance took place in the stone blocks or sleepers supporting them, the flat base of the chair upon which the rails rested being tilted by unequal subsidence, the end of one rail became depressed, while that of the other was elevated. Hence constant jolts and shocks, the reaction of which very often caused the fracture of the rails, and occasionally threw the engine off the road. [Illustration: HALF-LAP JOINT.] To remedy this imperfection, Mr. Stephenson devised a new chair, with an entirely new mode of fixing the rails therein. Instead of adopting the _butt-joint_ which had hitherto been used in all cast-iron rails, he adopted the _half-lap joint_, by which means the rails extended a certain distance over each other at the ends like a scarf-joint. These ends, instead of resting on the flat chair, were made to rest upon the apex of a curve forming the bottom of the chair. The supports were also extended from three feet to three feet nine inches or four feet apart. These rails were accordingly substituted for the old cast-iron plates on the Killingworth Colliery Railway, and they were found to be a very great improvement on the previous system, adding both to the efficiency of the horse-power (still used on the railway) and to the smooth action of the locomotive engine, but more particularly increasing the efficiency of the latter. This improved form of the rail and chair was embodied in a patent taken out in the joint names of Mr. Losh, of Newcastle, iron founder, and of Mr. Stephenson, bearing date the 30th of September, 1816. Mr. Losh being a wealthy, enterprising iron-manufacturer, and having confidence in George Stephenson and his improvements, found the money for the purpose of taking out the patent, which in those days was a very costly as well as troublesome affair. At the same time, Mr. Losh guaranteed Stephenson a salary of £100 per annum, with a share in the profits arising from his inventions, conditional on his attending at the Walker Iron-works two days a week--an arrangement to which the owners of the Killingworth Colliery cheerfully gave their sanction. [Illustration: OLD KILLINGWORTH LOCOMOTIVE STILL IN USE.] The specification of 1816 included various important improvements in the locomotive itself. The wheels of the engine were improved, being altered from cast to malleable iron, in whole or in part, by which they were made lighter as well as more durable and safe. The patent also included the ingenious and original contrivance by which the steam generated in the boiler was made to serve as a substitute for springs--an expedient already explained in a preceding chapter.