64 Thinys not generally Known. 



northern deserts of Africa. Thus Captain Sturt, in his account 

 of his Australian exploration, says : " The ground was almost 

 a molten surface ; and if a match accidentally fell upon it, it 

 immediately ignited." Sir John Herschel has observed the 

 temperature of the surface soil in South Africa as high as 159 

 Fahrenheit. An ordinary lucifer-match does not ignite when 

 simply pressed upon a smooth surface at 212 ; but in the act 

 of withdrawing it it takes fire, and the slightest friction upon 

 such a surface of course ignites it. 



HOW DR. WOLLASTON COMPARED THE LIGHT OF THE SUN AND 

 THE FIXED STARS. 



In order to compare the Light of the Sun with that of a 

 Star, Dr. Wollaston took as an intermediate object of compa- 

 rison the light of a candle reflected from a bulb about a quarter 

 of an inch in diameter, filled with quicksilver ; and seen by one 

 eye through a lens of two inches focus, at the same time that 

 the star on the sun's image, placed at a proper distance, was 

 viewed by the other eye through a telescope. The mean of 

 various trials seemed to show that the light of Sirius is equal 

 to that of the sun seen in a glass bulb -^th of an inch in dia- 

 meter, at the distance of 210 feet ; or that they are in the 

 proportion of one to ten thousand millions : but as nearly one 

 half of this light is lost by reflection, the real proportion be- 

 tween the light from Sirius and the sun is not greater than 

 that of one to twenty thousand millions. 



" THE SUN DARKENED." 



Humboldt selects the following example from historical 

 records as to the occurrence of a sudden decrease in the light 

 of the Sun : 



A.D. 33, the year of the Crucifixion. "Now from the sixth hour 

 there was darkness over all the land till the ninth hour" (St. Matthew 

 xxvii. 45). According to St. Luke (xxiii. 45), " the sun was darkened.' 1 

 In order to explain and corroborate these narrations, Eusebius brings 

 forward an eclipse of the sun in the 2J2d Olympiad, which had been 

 noticed by the chronicler Phlegon of Tralles (Ideler, Handluch der 

 Mathem. Chronologic, Bd. ii. p. 417). Wurn, however, has shown that 

 the eclipse which occurred during this Olympiad, and was visible over 

 the whole of Asia Minor, must have happened as early as the 24th of 

 November 29 A.D. The day of the Crucifixion corresponded with the 

 Jewish Passover (Ideler, Bd. i. pp. 515-520), on the 14th of the month 

 Nisan, and the Passover was always celebrated at the time of the full 

 moon. The sun cannot therefore have been darkened for three hours by 

 the moon. The Jesuit Scheiner thinks the decrease in the light might 

 be ascribed to the occurrence of large sun-spots. 



THE SUN AND TERRESTRIAL MAGNETISM. 



The important influence exerted by the Sun's body, as a 



