62 ANNUAL OF SCIENTIFIC DISCOVERY. 



must be sonofht why steam taken from one part is able to over- 

 come the resistance o])posed to its entrance in another part of the 

 same boiler. If a pipe conveying steam were turned directly 

 back into the water of the same boiler, it is evident that equilib- 

 rium would ensue and no effect be produced. If, on the other 

 hand, a break were made in the continuity of the pipe, so as to 

 leave an interval open to the atmosphere, steam would rush from 

 one pipe and water from the other in the boiler with a velocity 

 proportioned to their different densities. In constructing the in- 

 jector, the feed- water chamber is placed at the break in the pipe, 

 and this arrangement accounts for the power of the steam to over- 

 come the resistance to its entrance into the receiving-pipe of the 

 boiler. The jet of steam, being concentrated on the water, forces 

 its way through the interval surrounded by feed water, by contact 

 with which it is graduall}- condensed, and reduced in volume and 

 velocity, until it is entirely converted into water at the throat. 

 In doing so, it imparts to the feed water a velocity proportioned 

 to the pressure in the boiler and its own temperature; and, the 

 water being non-elastic, it acquires sufficient momentum to over- 

 come the resistance in the water space of the boiler." — Scieniijic 

 American. 



LOCOMOTIVES FOR COMMON ROADS. 



Mr. R W. Thomson, of Edinburgh, has recently patented a 

 locomotive engine for common roads which promises more success 

 than any other previously invented. Efforts to bring stenm into 

 play on ordinary roads long preceded the introduction of railway's, 

 the latter of which were used from the uniform failure of engi- 

 neers to devise a steam carriage capable of running on common 

 roads. Even before the time of Watt attempts of this kind were 

 made. The main difficulty was that the great weight caused the 

 W'heels to sink into some kinds of roads, and to slip on others; 

 the whole difficulty lies in the wheels. In Boydei's endless rail- 

 way there was a common road engine with very large wheels, 

 round the tires of which were loosely fastened short lengths of 

 rails, secured to Hat sleepers. As the wheels went slowly round, 

 these pieces came down in succession in front of the wheels, 

 which thus had a kind of very ill-laid railway to travel over. 

 This ugly-looking machine travelled very slowly, and its costly and 

 complex mechanism was soon put out of order; its only merits 

 were that it did not injure the roads, nor did the wheels slip. In 

 Bray's plan the tires were smooth, without any permanent pro- 

 jections; but, in order to secure the necessary hold on the roatl, 

 there were claws projected and retracted through the tires, oper- 

 ated by ingenious machinery; the claws were soon worn out, and 

 the road punched full of holes. Wheels with tlat teeth riveted on 

 the tires take a good liold of the ground, but are very destructive 

 to roads, while smooth wheels slip; so that until wheels can be 

 made that will neitlier slip nor destroy roads, steam carriages for 

 commcm roads cannot come into o^eneral use. 



In Mr. Thomson's engine the tires are made of bands of 



