66 Things not generally Known. 



sun shines for six months together on one Pole, and the other 

 six months on the other Pole. 



HOW THE DISTANCE OF THE SUN IS ASCERTAINED BY THE 

 YARD-MEASURE. 



Professor Airy, in his Six Lectures on Astronomy, gives a 

 masterly analysis of a problem of considerable intricacy, viz. 

 the determination of the parallax of the sun, and consequently 

 of his distance, by observations of the transit of Venus, the con- 

 necting link between measures upon the earth's surface and the 

 dimensions of our system. The further step of investigating 

 the parallax, and consequently the distance of the fixed stars 

 (where that is practicable), is also elucidated; and the author, 

 with evident satisfaction, thus sums up the several steps : 



By means of a yard-measure, a base-line in a survey was measured ; 

 from this, by the tria.ngulations and computations of a survey, an arc of 

 meridian on the earth was measured ; from this, with proper observa- 

 tions with the zenith sector, the surveys being also repeated on different 

 parts of the earth, the earth's form and dimensions were ascertained ; 

 from these, and a previous independent knowledge of the proportions of 

 the distances of the earth and other planets from the sun, with observa- 

 tions of the transit of Venus, the sun's distance is determined ; and from 

 this, with observations leading to the parallax of the stars, the distance 

 of the stars is determined. And every step in the process can be dis- 

 tinctly referred to its basis, that is, the yard-measure. 



HOW THE TIDES ARE PRODUCED BY THE SUN AND MOON. 



Each of these bodies excites, by its attraction upon the 

 waters of the sea, two gigantic waves, which flow in the same 

 direction round the world as the attracting bodies themselves 

 apparently do. The two waves of the moon, on account of 

 her greater nearness, are about 3| times as large as those ex- 

 cited by the sun. One of these waves has its crest on the 

 quarter of the earth^s surface which is turned towards the 

 moon ; the other is at the opposite side. Both these quarters 

 possess the flow of the tide, while the regions which lie be- 

 tween have the ebb. Although in the open sea the height of 

 the tide amounts to only about three feet, and only in certain 

 narrow channels, where the moving water is squeezed together, 

 rises to thirty feet, the might of the phenomenon is never- 

 theless manifest from the calculation of Bessel, according to 

 which a quarter of the earth covered by the sea possesses dur- 

 ing the flow of the tide about 25,000 cubic miles of water 

 more than during the ebb ; and that, therefore, such a mass of 

 water must in 6 hours flow from one quarter of the earth to 

 the other. Professor Helmhottz. 



