384 THE PRINCIPLES OF SCIENCE. 



water, and mercury, the temperature of the maximum 

 density of water (39'ioi Fahr. or 4-o Cent.), the latent 

 heats of water and steam, the boiling-point of water 

 under standard pressure, the melting and boiling-points 

 of mercury, and so on. 



Astronomical Constants. 



The third great class consists of numbers possessing far 

 less generality because they refer, not to the universal 

 properties of matter, but to the special forms and dis- 

 tances in which matter has been disposed in the part of 

 the universe open to our examination. We have, first of 

 all, to define the magnitude and form of the earth, its mean 

 density, the constant of aberration of light expressing the 

 relation between the earth's mean velocity in space and 

 the velocity of light. From the earth, as our observatory, 

 we then proceed to lay down the mean distances of the 

 sun, and of the planets from the same centre ; all the 

 elements of the planetary orbits, the magnitudes, densities, 

 masses, periods of axial rotation of the several planets 

 are by degrees determined with growing accuracy. The 

 same labours must be gone through for the satellites. 

 Catalogues of comets with the elements of their orbits, 

 as far as ascertainable, must not be omitted. 



From the earth's orbit as a new base of observations, 

 we next proceed to survey the heavens and lay down the 

 apparent positions, magnitudes, motions, distances, periods 

 of variation, &c. of the stars. All catalogues of stars from 

 those of Hipparchus and Tycho, are full of numbers ex- 

 pressing rudely the conformation of the visible universe. 

 But there is obviously no limit to the labours of astrono- 

 mers ; not only are millions of distant stars awaiting their 

 first measurements, but those already registered require 

 endless scrutiny as regards their movements in the three 



