45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 563 



VELOCITY OF LIGHT. 



In 1675 and 1676 Olaf Eoemer, a Danish astronomer, first determined the velocity of 

 Light from observations of the eclipses of Jupiter's first satellite as 192,500 miles per second 

 Bradley, at Kew, in 1723 determined the velocity of L'ght as 191,515 miles per second. 

 M. Foucault, by a modification of Wheatstone's revolving mirror, calculated the velocity 

 at 185,177 miles per second. 



M. Fizeau, at Paris, in 1849, by experiments carried on between Suresnes and 

 Montmartre, found the velocity to be 194,677 miles per second. 



More recent experiments by Foucault and Fizeau, 186,633 miles per second is stated. 

 By another experiment, the result is given as 196,000 miles per second. 

 The recent American determinations of the velocity of light, justly considered as of 

 far superior precision to any others yet executed, give the following results : 



Michelson at Naval Academy, in 1879 299,910 km. 



Michelson, at Cleveland, 1882 299,853 



Newcombe, at Washington, 1882, using only results supposed to 



be nearly free from constant errors 299,860 



Newcomb, including all determinations 299,810 



To these are added for comparison : 



Foucault, at Paris, in 1862 298,000 



Cornu, at Paris, in 1874 298,500 



Cornu, at Paris, in 1878 300,400 



The same discussed by Listing 299,990 



Young and Forbes, 1880-81 . 301,382 



Prof. Newcomb's finally-concluded result is that light travels in vacua at the rate of 

 299,860 - 30 kilometres per second. And the probable error of thirty kilometres, small as 

 it is, has been liberally estimated. Measures of the Velocity of Light made during the years 

 1881, 82, ly Professor 8. Newcomb, U.S. Navy. See "Nature," June 21th, 1886. 



VELOCITY OF SOUND. 



The velocity of Sound is 1,125' feet in a second, at a temperature of 61 Fahreheit, 

 or 16'6 C ; therefore a distance of 50 miles would be traversed in 4 minutes. 



In 1823, on two hills, Kooltjesberg and Zevenboomen, near Amsterdam, accurate 

 experiments were made to determine the velocity of sound by Moll and Van Beck. 

 Making corrections for temperature, moisture, barometric pressure, and the action of the 

 wind, the result of these experiments as corrected by Schroder Van der Kolk gives 1,092-78 

 feet as the velocity of sound per second in a dry atmosphere with the barometer reading 

 760 m.m., or 29-931 inches. 



The velocity of sound increases with increase of temperature : this amounts to nearly 

 two feet for every degree Centigrade. For the same temperature it is independent of the 

 density of the air, and therefore of the pressure. 



VELOCITY OF ELECTRICITY. 



" This is so great that the most rapid motion that can be produced by art appears 

 to be actual rest when compared with it. A wheel revolving with a rapidity sufficient to 

 render the spokes invisible, when illuminated by a flash of electricity, is seen for an instant 

 with all its spokes distinct as if it were in a state of absolute repose ; because, however 

 rapid the rotation may be, the light has come and already ceased before the wheel has had 

 time to turn through a sensible space. 



" Let a circular piece of pasteboard be divided into three sections ; let one be painted Hue 

 another yellow, and a third red. Cause it to rotate rapidly, it will appear white, because a 

 sunbeam consists of a mixture of these colours, and the rapidity of the motion causes the 

 distinction of colours to be lost to the eye ; but the instant the pasteboard is illuminated by 

 the electric spark, it seems to stand still, and each colour appears as distinct as if the disc 

 were at rest. 



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