NATURAL PHILOSOPHY. 165 



power of a horse. The commercial question, of what a manufacturer 

 should give as a horse-power, could not be discussed ; for the actual 

 power was only a small element in the actual cost of an engine, that 

 varying with every peculiar application of the machine. The surplus 

 power now given by manufacturers has evidently arisen from a more 

 perfect machine being now produced, by the use of tools in the manu- 

 facture, the introduction of metallic rings instead of hemp packing, 

 more perfect valves, and numerous modifications, all of which were 

 apart from, and independent of, the question of the original standard, 

 Avhich, it was admitted, could not be improved, and should not, there- 

 fore, be altered. 



ILLUSTRATIONS OF LOCOMOTIVE SPEED. 



DR. LARDXSR, in his lately published Economy of Railways, thus en- 

 deavors to convey to the unpractised reader the enormous speed of 

 a locomotive going at the rate of seventy miles an hour : " Seventy 

 miles an hour is, in round numbers, 105 feet per second ; that is, a 

 motion in virtue of which a passenger is carried over thirty-five yards 

 between two beats of a common clock. Two objects near him, a yard 

 asuuder, pass by his eye in the 35th part of a second ; and if thirty- 

 five stakes were erected by the side of the road, one yard asunder, the 

 whole would pass his eye between two beats of a clock ; if they had 

 any strong color, such as red, they would appear a continuous flash of red. 

 At such a speed, therefore, the objects on the side of the road are not 

 distinguishable. When two trains, having this speed, pass each other, 

 the relative velocity will be double this, or seventy yards per second, 

 and if one of the trains were seventy yards long, it would flash by in a 

 single second. To accomplish this, supposing the driving-wheels seven 

 feet in diameter, the piston must change its direction in the cylinder 

 ten times in a second. But there are two cylinders, and the mechanism 

 is so regulated that the discharges of steam are alternate. There are, 

 therefore, twenty discharges of steam per second, at equal intervals ; 

 and thus these twenty puffs divide a second into twenty equal parts, 

 each puff having the twentieth of a second between it and that which 

 precedes and follows it. The ear, like the eye, is limited in the rapidity 

 of its sensations ; and, sensitive as that organ is, it is not capable of 

 distinguishing sounds which succeed each other at intervals of the 

 twentieth part of a second. According to the experiments of Dr. Hut- 

 ton, the flight of a cannon ball was 6700 feet in one quarter of a minute, 

 equal to five miles per minute, or 300 miles per hour. It follows, there- 

 fore, that a railway train, going at the rate of 75 miles per hour, has a 

 velocity of one fourth that of a cannon ball ; and the momentum of 

 such a mass, moving at such a speed, is equivalent to the aggregate 

 force of a number of cannon balls equal to one fourth of its own weight." 





