This second engine (Fig. 51) was also fitted with two vertical cylinders, _C c_, but the connecting-rods were attached directly to the four driving-wheels, _W W´_. To permit the necessary freedom of motion, "ball-and-socket" joints were adopted, to unite the rods with the cross-heads, _R r_, and with the cranks, _R´ Y´_; and the two driving-axles were connected by an endless chain, _T t´_. The cranked axle and the outside connection of the wheels, as specified in the patent, were not used until afterward, it having been found impossible to get the cranked axles made. In this engine the forced draught obtained by the impulse of the exhaust-steam was adopted, doubling the power of the machine and permitting the use of coke as a fuel, and making it possible to adopt the multi-tubular boiler. Small steam-cylinders, _S S S_, took the weight of the engine and served as springs. It was at about this time that George Stephenson and Sir Humphry Davy, independently and almost simultaneously, invented the "safety-lamp," without which few mines of bituminous coal could to-day be worked. The former used small tubes, the latter fine wire gauze, to intercept the flame. Stephenson proved the efficiency of his lamp by going with it directly into the inflammable atmosphere of a dangerous mine, and repeatedly permitting the light to be extinguished when the lamp became surcharged with the explosive mixture which had so frequently proved fatal to the miners. This was in October and November, 1815, and Stephenson's work antedates that of the great philosopher.[52] The controversy which arose between the supporters of the rival claims of the two inventors was very earnest, and sometimes bitter. The friends of the young engineer raised a subscription, amounting to above £1,000, and presented it to him as a token of their appreciation of the value of his simple yet important contrivance. Of the two forms of lamp, that of Stephenson is claimed to be safest, the Davy lamp being liable to produce explosions by igniting the explosive gas when, by its combustion within the gauze cylinder, the latter is made red-hot. Under similar conditions, the Stephenson lamp is simply extinguished, as was seen at Barnsley, in 1857, at the Oaks Colliery, where both kinds of lamp were in use, and elsewhere. [52] _Vide_ "A Description of the Safety-Lamp invented by George Stephenson," etc., London, 1817.