DISCOVERT BY MEANS OF NEW EXPERIMENTS. 527 



pendulum, and allowing them to vibrate for considerable 

 periods of time, discovered that the attraction of the 

 earth was not sensibly aifected in different degrees by 

 different kinds of matter. It was by means of experi- 

 ments that Savart discovered that a high note of a given 

 degree of intensity was inaudible, not because of its pitch, 

 but because of its feebleness. By allowing the edge of a 

 fixed card to be struck by the teeth of a rotating wheel, 

 he found that with wheels of larger size higher notes 

 could be heard, because the teeth being farther apart, the 

 blows were stronger and more separate. 1 Biot also dis- 

 covered, by actual trial, that the lowest whispers could be 

 heard through 3120 feet in length of the cylindrical tubes 

 of the aqueduct of Paris. 



Baptista Porta, in 1560, discovered the principle of the 

 camera-obscura, and states that he found it by allowing 

 the sun to shine brightly through a very small hole in the 

 shutter of a dark room, and covering the hole with trans- 

 parent pictures. Kircher, about the year 1610, first made 

 the magic-lantern, by combining Porta's camera and paint- 

 ings with the light of a lamp. Newton's celebrated 

 discovery of the unequal refrangibility of differently 

 coloured rays of light, made about the year 1670, was 

 chiefly a result of devising and carrying out well-conceived 

 experiments. It was by inventing the use of a slit instead 

 of a round hole in these experiments, that Dr. Wollaston, 

 in the year 1 802, was led to discover the existence of seven 

 dark bands in the solar spectrum. Fraunhofer also, in the 

 year 1814, by examining the light of the sun, moon, and 

 Venus, in a similar way, found that the lines of each were 

 the same ; but with the light of the fixed stars he dis- 

 covered a difference, their dark lines were in different 



1 Mrs. Somerville, Connexion of the Physical Sciences, p. 153. 



