The Science record; a compendium of scientific progress and discovery . oonssurface, resembling snow in many parts, be explained bythe fact that the extinct volcanoes of our satellite are cov-ered with crystals of salt ? Any person who is accustomed to view the moonthrough a telescope must, I think, have been struck withthe dazzling snow-like appearance of the not an explanation of this be deduced from theexperiences gained by the last eruption of Vesuvius ?The burning lava in that instance retained an enormous ASTRONOMY. 559 quantity of salt, which did not escape until the mass
The Science record; a compendium of scientific progress and discovery . oonssurface, resembling snow in many parts, be explained bythe fact that the extinct volcanoes of our satellite are cov-ered with crystals of salt ? Any person who is accustomed to view the moonthrough a telescope must, I think, have been struck withthe dazzling snow-like appearance of the not an explanation of this be deduced from theexperiences gained by the last eruption of Vesuvius ?The burning lava in that instance retained an enormous ASTRONOMY. 559 quantity of salt, which did not escape until the mass be-came cool, when the whole stretch of lava emitted in 1872was covered with a crust of this substance. THE TRANSIT OF VENUS IN year 1874 is a very notable year in the history ofscience, for in it, on the 9th of December, will occur thephenomenon known to astronomers as the transit of Ve-nus. More than one hundred j^ears have elapsed since thelast occasion of this transit; another will happen in 1882,for, according to the laws which govern the respective mo-. I.—PATHS OF THE PLANET 874 AND tions of Venus and the earth, the transits, when they dohappen, occur in couples at comparatively short intervals ;but there will then be no other transit until the year phenomenon alluded to is the passage of the planetVenus between the earth and the sun, in such a positionwith regard to the earths orbit, that Venus is seen tomove like a round black spot over the suns face. Theimportance of this phenomenon, in a scientific point ofview, may be judged from the fact that it affords astrono- 560 SCIENCE RECORD. mers the best means of measuring the distances of theheavenly bodies, and of ascertaining their weight and di-mensions. The first occasion on which a transit of Venus wasobserved for this purpose was in 1761, the eminent astro-nomer, Dr. Halley, having recommended the method, anddevised a plan of operation to be used after his death, ashe knew he could not live until
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