374 ^ Short History of Astronomy [CH. xin. 



to the Royal Society in 1830-37, and William Whewell 

 (1794-1866), whose papers on the subject appeared between 

 1833 and 1851. Airy ( 281), then Astronomer Royal,, 

 also published in 1845 an important treatise dealing with 

 the whole subject, and discussing in detail the theory of 

 tides in bodies of water of limited extent and special form. 

 The analysis of tidal observations, a large number of which 

 taken fro-n all parts of the world are now available, has 

 subsequently been carried much further by new methods 

 due to Lord Kelvin and Professor G. H. Darwin. A 

 large quantity of information is thus available as to the 

 way in which tides actually vary in different places and 

 according to different positions of the sun and moon. 



Of late years a good deal of attention has been paid to 

 the effect of the attraction of the sun and moon in producing 

 alterations analogous to oceanic tides in the earth itself. 

 No body is perfectly rigid, and the forces in question must 

 therefore produce some tidal effect. The problem was first 

 investigated by Lord Kelvin in 1863, subsequently by 

 Professor Darwin and others. Although definite numerical 

 results are hardly attainable as yet, the work so far carried 

 out points to the comparative smallness of these bodily 

 tides and the consequent great rigidity of the earth, a result 

 of interest in connection with geological inquiries into the 

 nature of the interior of the earth. 



Some speculations connected with tidal friction are 

 referred to elsewhere ( 320). 



293. The series of propositions as to the stability of 

 the solar system established by Lagrange and Laplace 

 (chapter XL, 244, 245), regarded as abstract propositions 

 mathematically deducible from certain definite assumptions, 

 have been confirmed and extended by later mathematicians 

 such as Poisson and Leverrier ; but their claim to give 

 information as to the condition of the actual solar system 

 at an indefinitely distant future time receives much less 

 assent now than formerly. The general trend of scientific 

 thought has been towards the fuller recognition of the 

 merely aoproximate and probable character of even the best 

 ascertained portions of our knowledge ; " exact," "always," 

 and "certain" are words which are disappearing from the 

 scientific vocabulary, except as convenient abbreviations. 



