| Show MEASURING THE I SPEED OF LIGHT By Lawrence B Fletcher Light travels at the rato of about ono hundred and eightysix thousand miles a second and reaches the earth In a lltlle moro than eight minutes after It has started from the sun Now as the earth Is only a little ball less than eight thousand thou-sand miles In diameter it la clear that light will only occupy a very small part of a second In passing over any distance that can be measured on earth And yet light has been timed accurately not merely over a com so of a few miles but over one of a few yards The two methods meth-ods by which this has been done were Invented by two French scientists half a century ago To Illustrate the first method meth-od suppose you throw a ball straight up through a hole In a roof which hole Is opened and shut once every ten seconds Xow if I the ball finds the hole open when It goes up and If It Is thrown with such force that Its flight lasts just ten seconds It will find the hole upon again when It comes down and will return to your hand The name thing will happen If the balls flight last twenty thirty or forty seconds But if It Is lu tho air only five seconds or juat fifteen or twentylive It will hind the hole closed on Its return and will bo stopped Now In M Flzeaus measurement of the speed of light a sunbeam was ncnt through a small hole which was open and shut bv a toothed wheel rotating In front of It Each tooth as It passed the hole cutoff cut-off the light and the section of sunbeam that passed between two teeth corresponded corres-ponded to the bull In our Illustration As tho wheel kept on revolving there TVHS a whole series of these balls but that made no difference They went ve or six miles to a mirror which reflected or sent them back again On the return trip if rri LOST STRAYED OR STOLEN v s of I I I I I r a1 + n It rsi II ft 1 r r I 1 r 1 I u t r1ara r i I u I I I 1 I ti t I I SI r ir 7 1 r vli t r So C HPWPWtr t Said Mistress Colo Why bless my soul I My Mdo is lost Im afraid o I bade him stay near menow didnt he hear me c I Or has he gone home with the moid the speed of the wheel 1 was made right each piece of sunbeam found a tooth instead in-stead of the space between the two teeth through which it had started out and was utoppcd so that the observer san darkness dark-ness so to speak Instead of light To produce this result it was found necessary to rotate thc wheel which had 710 teeth more than twelve times a second That Is to say the light went out to the mirror and back about eleven miles In the time In I which the edge of the wheel moved half the distance between the two teeth about the elghtccnthouaandth part of a second The second experimenter Foucalt used no toothed wheel but two mirrors only thirteen feet apart The first mirror re llcctcd a beam of light to the second which sent It back to The Hrst by which It was again reflected off along Its orig Inn I path but in the opposite direction Arrangements were made for observing the returning beam and making Its exact ex-act position Then the first minor was made to rotate several hundred times a second The returning re-turning beam was found to have moved the amount of the motion depending upon the speed of tho mirror The reason is I simply that the mirror Itself had moved through a small angle while the light was I going out to the fixed mirror and back This anglo could be calculated from the r motion of the reflected beam and as the speed of tho mirror was known It way possible to calculate the very small fiac ion about a fortymillionth of a J snc mum which light occupies in traveling but II 1 twentysix feet l Indeed by putting a tube filled with I water In the path of the beam Foitcalt was able to prove that light travels moro 1 slowly through water than through air I The speed of light has since been re measured a number of time by both oC these methods with Improved apparatus and Is now known very accurately yet so 1 careful are sclontllic men about such I things that another measurement Is now being made In France y I |
