Scr. XXXVIII. CONCLUSIUX. 389 



pheuomenH, is indispensable for acquiring a knowledge 

 of the revolutions of the celestial bodies and their recip- 

 rocal influences. The motions of the satellites are af- 

 fected by the forms of their primaries, and the figures 

 of -the planets themselves depend upon their rotations. 

 The symmetry of their internal structure proves the 

 stability of these rotatory motions, and the immutability 

 of the length of the day, which furnishes an invariable 

 standard of time ; and the actual size of the terrestrial 

 spheroid affords the means of ascertaining the dimensions 

 of the solar system, and provides an invariable founda- 

 tion for a system of weights and measures. The mutual 

 attraction of the celestial bodies disturbs the fluids at 

 their surfaces, whence the theory of the tides and of the 

 oscillations of the atmosphere. The density and elas- 

 ticity of the air, varying with every alternation of tern-' 

 perature, lead to the consideration of barometrical 

 changes, the measurement of heights, and capillary at- 

 traction ; and the doctrine of sound, including the theory 

 of music, is to be referred to the small undulations of 

 the aerial medium. A knowledge of the action of mat- 

 ter upon light is requisite for tracing the curved path of 

 its rays through the atmosphere, by which the true 

 places of distant objects are determined whether in the 

 heavens or on the earth. By this we learn the nature 

 and properties of the sunbeam, the mode of its propaga- 

 tion through the ethereal fluid, or in the interior of ma- 

 terial bodies, and the origin of color. By the eclipses of 

 Jupiter's satellites, the velocity of light is ascertained ; and 

 that velocity, in the aberration of the fixed stars, fur- 

 nishes the only direct proof of the real motion of the 

 earth. The effects of the invisible rays of light are im- 

 mediately connected with chemical action ; and heat, 

 forming a part of the solar ray so essential to animated 

 and inanimated existence, whether considered as invisi- 

 ble light or as a distinct quality, is too important an agent 

 in the economy of creation, not to hold a principal place 

 in the connection of physical sciences. Whence follows 

 its distribution in the interior and over the surface of the 

 globe, its power on the geological convulsions of our 

 planet, its influence on the atmosphere and on climate, 

 and its effects on vegetable and animal life, evinced in 

 K K 2 



