549 



SCIENTIFIC SIDE-LIGHTS 



Prayer 

 Precision 



2 7 GO. PRECISION OF SCIENCE Mi- 

 nute Exactness of Measurement Led to Dis- 

 covery of Velocity of Light The Satellites 

 of Jupiter. Romer watched this moon [of 

 Jupiter], saw it move round in front of the 

 planet, pass to the other side of it, and then 

 plunge into Jupiter's shadow, behaving like 

 a lamp suddenly extinguished; at the second 

 edge of the shadow he saw it reappear, like 

 a lamp suddenly lighted. The moon thus 

 acted the part of a signal-light to the as- 

 tronomer, and enabled him to tell exactly 

 its time of revolution. The period between 

 two successive lightings-up of the lunar 

 lamp he found to be 42 hours, 28 minutes, 

 and 35 seconds. This measurement of time 

 was so accurate that, having determined the 

 moment when the moon emerged from the 

 shadow, the moment of its hundredth ap- 

 pearance could also be determined. In fact, 

 it would be 100 times 42 hours, 28 minutes, 

 35 seconds after the first observation. Ro- 

 mer's first observation was made when the 

 earth was in the part of its orbit nearest 

 Jupiter. About six months afterwards, 

 the earth being then at the opposite side 

 of its orbit, when the little moon ought 

 to have made its hundredth appearance, it 

 was found unpunctual, being fully 15 min- 

 utes behind its calculated time. Its ap- 

 pearance, moreover, had been growing gradu- 

 ally later as the earth retreated towards 

 the part of its orbit most distant from Ju- 

 piter. Romer reasoned thus : " Had I been 

 able to remain at the other side of the 

 earth's orbit the moon might have appeared 

 always at the proper instant; an observer 

 placed there would probably have seen the 

 moon 15 minutes ago, the retardation in 

 my case being due to the fact that the light 

 requires 15 minutes to travel from the place 

 where my first observation was made to my 

 present position." 



This flash of genius was immediately 

 succeeded by another. " If this surmise 

 be correct," Romer reasoned, " then as I 

 approach Jupiter along the other side of 

 the earth's orbit the retardation ought 

 to become gradually less, and when I reach 

 the place of my first observation there ought 

 to be no retardation at all." He found this 

 to be the case, and thus not only proved that 

 light required time to pass through space, 

 but also determined its rate of propagation. 

 TYNDALL Lectures on Light, lect. 1, p. 20. 

 (A., 1898.) 



2707. 



Observation Needs 



Correction by Deeper Knowledge The Ap- 

 parent Not the Real. Direct observation 

 furnishes only what has been called the "raw 

 material " of the positions of the heavenly 

 bodies. A number of highly complex cor- 

 rections have to be applied before their mean 

 can be disengaged from their apparent places 

 on the sphere. Of these, the most consider- 

 able and familiar is atmospheric refraction, 

 by which objects seem to stand higher in 

 the sky than they in reality do, the effect 



being evanescent at the zenith, and attain- 

 ing, by gradations varying with conditions 

 of pressure and temperature, a maximum at 

 the horizon. Moreover, the points to which 

 measurements are referred are themselves 

 in motion, either continually in one direc- 

 tion, or periodically to and fro. The pre- 

 cession of the equinoxes is slowly progress- 

 ive, or rather retrogressive; the nutation 

 of the pole oscillatory in a period of about 

 eighteen years added to which, the non- 

 instantaneous transmission of light, com- 

 bined with the movement of the earth in 

 its orbit, causes a minute displacement 

 known as aberration. Now it is easy to see 

 that any uncertainty in the application of 

 these corrections saps the very foundations 

 of exact astronomy. CLERKE History of As- 

 tronomy, pt. i, ch. 2, p. 37. (Bl., 1893.) 



27O8. Telegraphic Nota- 

 tion of Time Buoy of Sunken Cable Found 

 on the High Sea. To show what could be 

 done if there were perfect means of de- 

 termining the time, the following narrative 

 may be cited: When the "Great Eastern" 

 is carrying a telegraph cable across the 

 Atlantic, her captain, of course, knows the 

 true Greenwich time within a single second, 

 for it is flashed to him from Valentia. He 

 can therefore determine his true place with 

 great accuracy. Now it chanced that on 

 one occasion the captain of the " Great 

 Eastern," while thus in telegraphic com- 

 munication with Greenwich through Valen- 

 tia, had occasion to search for a buoy which 

 had been left floating (attached to a sunk 

 cable) in a particular latitude and longi- 

 tude. He made for the spot according to 

 his calculated latitude and longitude, and 

 (according to the account) after the final 

 directions had been given to the effect that 

 the ship should fojlow a certain course for 

 a certain time he went below to examine 

 a chart. When the time came he was about 

 to go on deck, hoping to have made his 

 course so truly that the buoy would be in 

 sight; but at that very instant the ship's 

 side was struck by the buoy. PROCTOR Ex- 

 panse of Heaven, p. 34. (L. G. & Co., 1897.) 



27O9. . Volcanic Dust of 



Iceland, Fallen in Nonuay, Identified in 

 Germany. We sometimes meet with this 

 far-traveled, volcanic dust under very un- 

 expected circumstances. Thus, in the 

 spring of 1875 I had occasion to visit Pro- 

 fessor Vom Rath, of Bonn, who showed me 

 a quantity of fine volcanic dust which had 

 during the past winter fallen in considerable 

 quantities in certain parts of Norway. This 

 dust, upon microscopic examination, proved 

 to be so similar to what was known to be 

 frequently ejected from the Icelandic vol- 

 canoes that a strong presumption was raised 

 that volcanic outbursts had been going on 

 in that island. On returning to England 

 I found that the first steamer of the season 

 had just reached Leith from Iceland, bring- 

 ing the intelligence that very violent erup- 



