PICARD' s LUMINOUS BAROMETER. 453 



found a short note, barely filling a printed page, which 

 contains the suggestion which was the original cause 

 which started the whole scientific world to puzzling over 

 the wonders of the electric light. 



The terrestrial measurements which enabled Newton to 

 correct his calculations concerning the moon and to verify 

 his belief in the effect of the earth's gravity thereon, were 

 made by Jean Picard, a priest and an astronomer of re- 

 markable ability. It was Picard who informed the Royal 

 Academy of a curious effect which he had observed in the 

 barometer which he employed in the Paris Observatory. 

 The instrument of that time was merely a glass tube 

 closed above, open below, exhausted of air and inserted, 

 open end downwards, in a cup of mercury: the metal, of 

 course, rising in the tube under the atmospheric pressure. 

 Picard observed that when the instrument itself was 

 moved so as to cause the mercury to vibrate in the tube, a 

 light appeared in the empty portion of the latter, clearly 

 visible in the dark, It is said that he first saw it while 

 carrying the apparatus in his hands from one part of the 

 observatory to another after nightfall. At all events, there 

 was no mistaking the luminosity which was a sort of 

 broken glow above the quicksilver, and which appeared 

 best when the mercury descended quickly. The note, 

 which bears the date of ^67^ adds that efforts had been 

 made (combined experiments, probably) to find other 

 barometers which would behave similarly, but not one 

 had been encountered; that it had been resolved to ex- 

 amine the matter in every possible way, and that the 

 future discoveries would be set forth in detail. 1 The 

 same cheerful confidence which the king had shown con- 

 cerning coming developments in general, is here reflected 

 with regard to what was going to be found out about this 

 singular light. 



'Mem. de 1'Acad. Roy. des Sciences. Paris, 1730, vol. x., p. 556. 



