84 MECHANICS. 



occasions the rarefied air thus produced to expand 

 about 1000 times the space of the gunpowder.* 



Range of Shot, Rockets, and Bombs. 



The range of Congreve's rockets is 3,300 yards, 

 or nearly two miles. 



The large mortar in the park is said to have thrown 

 a 13-inch bomb-shell, filled with lead, above three 

 miles. 



The mean vertical ascent of a great number of 

 common rockets, discharged at Woolwich for the 

 purpose of ascertaining the comparative strength of 

 gunpowder, was 480 yards. 



Horse Power. 



The power of a horse is understood to be that 

 which will elevate a weight of 33,000f pounds, the 

 height of one foot in a minute of time, equal to about 

 90 pounds at the rate of four miles per hour. This 

 is a force greater than that exerted by a common 

 cart horse, which is not estimated at more than 70 

 pounds : that is to say, 



That a horse harnessed to a cart, weighing with its 

 load 40 cwt. or two tons, and drawing on a level 

 road at the rate of 4 miles an hour, makes use of the 

 same force as if his traces, instead of being fastened 

 to a cart, were passed over a pulley, and lifted per- 

 pendicularly a weight of 70 pounds. 



The expression of the power of the steam engine 

 in horses' powers, is consequently more practical 

 than scientific. It was introduced when steam en- 

 gines first began to supersede horse mills, when the 



* In blasting, a pound of gunpowder is allowed to a foot of 

 bore. 



t Another estimate reduces this to only 22,000 pounds, raised 

 one foot high in a minute, equivalent to 100 pounds 2f miles 

 per hour. 



