PHYSICAL ASTRONOMY. 17 



principles which are simple and well known. But the 

 different expedients for the transmission of the force 

 thus obtained, as well as for increasing the force in what 

 are called " high-pressure" engines, have greatly exer- 

 cised mechanical ingenuity, and immensely augmented 

 our stock of subordinate mechanism. No class of inven- 

 tions has occasioned so entire a change in the construc- 

 tion and application of machinery as this ; none has 

 tended more to improve and complete the main processes 

 in arts and manufactures ; none more to facilitate the 

 intercommunication between our manufacturing and 

 other great towns; and consequently, none has more 

 contributed to the commercial ascendancy of Great 

 Britain in the scale of nations. Yet all this flowed, in 

 the first instance, from an incident to which one can 

 hardly make a formal reference without exciting risible 

 emotions. 



Physical Astronomy) again, as a department of mathe- 

 matical theory, owes its origin to a simple accident. Sir 

 Isaac Newton was forced from Cambridge in the year 

 1666 by the plague. During his retirement, his friend 

 Dr. Pemberton states, as he was " sitting alone in a 

 garden, some apples falling from a tree, led his thoughts 

 to the subject of gravity." He was induced to conjec- 

 ture that the moon was retained in her orbit by the 

 same kind of force as that which caused the apples to 

 descend to the earth, or that gave a curvilinear motion 

 to bullets or other projectiles near the earth's surface. 

 Ere long, he made a computation upon this hypothesis ; 

 and found that the deviation of the moon, moving in her 

 orbit, from the tangent at any given point, was precisely 

 what it ought to be, supposing the force of terrestrial 

 gravity to vary inversely as the squares of the distances 



c 



