DISCOVERY BY MEANS OF NEW OBSERVATIONS. 565 



note to the preface of his ' Treatise on the Theory and 

 Practice of Perspective ' says : ' Since this work was 

 printed off I have seen a substance excellently adapted to 

 the purpose of wiping from paper the marks of a black- 

 lead pencil. It must therefore be of singular use to those 

 who practise drawing. It is sold by Mr. Nairne, mathe- 

 matical instrument maker to the Royal Exchange. He 

 sells a cubical piece of half an inch for three shillings. 

 He says it will last for years.' l 



a. By additional or new obserations with known 

 instruments or by known methods. Nearly all astro- 

 nomical discoveries have been made by this plan. By 

 the employment of the methods of observation then in use, 

 the astronomer Hipparchus, 150 years before Christ, dis- 

 covered the precession of the equinoxes. By observation it 

 was discovered that ' some stars are darker on one side than 

 upon the other ; Mira Ceti, for example, is invisible to the 

 naked eye during five months out of eleven.' 2 By means of 

 ordinary ; observations and calculations, Professor Newton, 

 of America, not very long ago concluded that 7^ millions 

 of meteors, visible to the unassisted eye, pass through the 

 atmosphere of the earth every 24 hours. ' Mersenne and 

 others had noticed (1636), that when a string vibrates, 

 one which is in unison with it vibrates without being 

 touched ; and that this was true if the second string was 

 an octave or a twelfth below the first.' 3 



It was by observation that Grrimaldi, in 1665, dis- 

 covered the phenomenon of diffraction or inflexion of 

 light. Erasmus Bartolinus, a physican of Copenhagen, 

 observed whilst making experiments with crystals of Ice- 

 land spar, that an inkspot appeared double when viewed 



1 The Telegraphic Journal, vol. v. p. 133. 



2 Whewell, Plurality of Worlds, p. 155. 



3 Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. p. 252 



