Hi report — 1865. 



where disco very, everywhere enjoyment, everywhere some useful and there- 

 fore some worthy result. Life in every form, of every grade, in every stage ; 

 man in every clime and under all conditions ; the life that now surrounds 

 us, and that which has passed away ; — -these subjects of high contemplation 

 have been examined often, if not always, in the spirit of that philosophy 

 which is slowly raising, on a broad security of observed facts, sure induc- 

 tions, and repeated experiments, the steady columns of the temple of physi- 

 cal truth. 



Few of the great branches of the study of nature on which modern philo- 

 sophy is intent were left unconsidered in the schools of Athens ; hardly one 

 of them was or indeed could be made the subject of accurate experiment. 

 The precious instruments of exact research — -the measures of time, and space, 

 and force, and motion — are of very modern date. If instead of the few 

 lenses and mirrors of which traces appear in Greek and Roman writers *, 

 there had been even the first Galilean or the smallest Newtonian telescope in 

 the hands of Hipparchus, Eratosthenes, or Ptolemy, would it have been left 

 to their remote successors to be still struggling with the elements of physical 

 astronomy, and waiting with impatience till another quarter of a century 

 shall have rolled away and given us one more good chance of measuring the 

 distance of the Sun by the transit of Venus ? Had such instruments as 

 Wheatstone's Chronoscope been invented, would it have been left to Foucault 

 to condense into his own apartment an experimental proof of the velocity of 

 light, and within a tract of thirty feet to determine the rate of its movement 

 through all the vast planetary space of millions and thousands of millions of 

 miles, more exactly than had been inferred by astronomers from observa- 

 tions of the satellites of Jupiter t ? By this experiment the velocity of light 

 appears to be less, sensibly less, than was previously admitted ; and this con- 

 clusion is of the highest interest. For, as by assuming too long a radius for 

 the orbit of Jupiter the calculated rate of light-mavement was too great ; so 

 now by employing the more exact rate and the same measures of time we 

 can correct the estimated distance of Jupiter and all the other planets from 

 the sun. We have in fact a really independent measure of planetary space ; 

 and it concurs with observations of the parallax of Mars, in requiring a con- 

 siderable reduction of the assumed diameters of the planetary paths. The 

 distance of the earth from the sun must be reduced from above ninety-five 

 to less than ninety-three millions of miles, and by this scale the other space- 

 measures of the solar system, excepting the diameter of the earth and the 

 distance and diameter of the moon, may be corrected +. 



* The effect of lenses or globes of glass or crystal (iJaXo?) in collecting the solar rays 

 to a point are familiarly referred to by Aristophanes in the Nubes, 76G ; and the orna- 

 mental use of convex and concave reflectors is known by the curious discussions in tile 

 IVth Book of Lucretius. 



t Fizeau performed experiments on the velocity of light between Suresnes and the 

 Butte Montmartre, by means of the oxyhydrogen light, reflected back in its own path. 



1 

 The space was 28,324 feet Engl. Twice this distance was traversed in 7iT7jn77 of a 



second = 167,528 geogr. miles in a second. From observations of Jupiter's satellites De- 

 lambre inferred 107,976 miles, Struve 166,096. The experiment of M. Foucault gives 

 298,000,000 metres = 160,920 geogr. miles. 



J Estimates of the earth's distance from the sun have varied much. Cassiniand 

 Flair steed, using observations of the parallax of Mars, ascribe to it 10,000 or 11,000 

 diameters of the earth = 79 or 89 millions of miles. Huyghens estimated it at 12,000 = 

 95 millions of miles. In 1745, Button reported it as the common opinion of astronomers 

 at 30 millions of leagues (Fr.)=90 millions miles (Engl.), but after the transit of Venus 



