344 THE PRINCIPLES OF SCIENCE. 



ment free from all the errors of construction, which affect 

 human instruments. The rotation of the earth, too, by 

 variously affecting the apparent velocity of ingress or 

 egress of Venus, as seen from different places, discloses 

 the amount of the parallax. It has been sufficiently 

 shown that by rightly choosing the moments of obser- 

 vation, the planetary bodies may often be made to reveal 

 their relative distance, to measure their own position, to 

 record their own movements with a high degree of ac- 

 curacy. With the improvement of astronomical instru- 

 ments, such conjunctions become less necessary to the 

 progress of the science, but it will always remain ad- 

 vantageous to choose those moments for observation 

 when instrumental errors enter with the least effect. 



In other sciences, exact quantitative laws can occasion- 

 ally be obtained without instrumental measurement, as 

 when we learn the exactly equal velocity of sounds of 

 different pitch, by observing that a peal of bells or a 

 musical performance is heard harmoniously at any dis- 

 tance to which the sound penetrates ; this could not be 

 the case, as Newton remarked, if one sound overtook 

 the other. One of the most important principles of the 

 atomic theory, was proved by implication, before the use 

 of the balance was introduced into chemistry. Wenzel 

 observed, before 1777, that when two neutral substances 

 decompose each other, the resulting salts are also neutral. 

 In mixing sodium sulphate and barium nitrate, we 

 obtain insoluble barium sulphate and neutral sodium 

 nitrate. This result could not follow unless the nitric 

 acid, requisite to saturate one atom of sodium, were 

 exactly equal to that required by one atom of barium, 

 so that an exchange could take place without leaving 

 either acid or base in excess d . 



A very important principle of mechanics may also be 



d Daubeny, ' Atomic Theory,' p. 30. 



