1809] ■^'^^ [Lowrie. 



Our scientific systems consist only of the elements from wliicli, and of 

 the order by which we construct them, and are, therefore, charged with 

 all the defects of our knowledge and constructive skill. In the early 

 stages of astronomy, many phenomena were treated as exceptional and 

 disturbing, which are now known to be normal pulsations of the vis viva 

 of the solar system, because now this force is better comprehended. And 

 the same is true of all growing sciences. All our scientific systems are 

 accustomed to discover that their ideals of nature are often very unreal, 

 and that the perturbations, which they attribute to nature, belong only 

 to themselves. Newly discovered facts or principles must always cause 

 some derangement or re-arrangement of the old furniture of the school 

 that admits them. 



Now that we know that the solar system is a part of a much grander 

 system, in which the sun itself revolves, we have a fact which is, in many 

 respects of great astronomical importance, and Avhich did not enter into 

 the inductions of former times ; and considering its character, it is not 

 unnatural to suppose that it is an essential element in all the motions of 

 the system. If this be so, then our whole system of astronomical 

 dynamics must, to some extent, open up to admit its influence and to 

 submit to such modifications as it may require. 



This fact can no more be without influence on the motions of the plan- 

 ets, than can be the revolution of the planets on their satellites. It ne- 

 cessarily made a great change in our knowledge of the form of planetary 

 orbits, though it may not greatly change our i-easoning about them. 

 And yet, what is the parallax of a star worth to us now, unless we know 

 whether the suu's motion (say 150,000,000 miles a year) was taken into 

 account, and whether the base of the parallactic angle was 190,000,000 

 + 75,000,000 miles, or 190,000,000-75,000,000, or some chord of inter- 

 mediate length ? How, without this, shall we value any ancient obser- 

 vation of the place of a star, or the record of stellar movements sup- 

 posed to be made in the construction of the pyramids ? It may now be 

 thought better to take one, or ten, or more years of the sun's motion 

 in the base of such an angle. 



Such changes in scientific theories do not often make any serious 

 changes in the laws which observation had discovered as facts, but rather 

 account for them, and show the common bond that unites them in nature 

 and in reason. When the centre of cosmical motion was transferred 

 from the earth to the sun, the laws of the solar system, as they had been 

 learned before, were not annulled. When light changed its base from 

 EMISSION to TJNDTJLATiOK, the laws of optics Were not seriously affected. 

 A law may be true as an expression of observed phenomena, even when 

 its principle is unknown or mistaken, or when it is erroneously supposed 

 to be itself a final and independent princii^le. 



I think the normal cause, not to speak of disturbing ones, of the re- 

 cession of the nodes can be found in the system or sub-system to which 

 the motion belongs, and that it is the same everywhere. It seems to me 

 to be a necessary consequence of the inclination of the dependent to the 



