Intelligence and Miscellaneous Articles, 315 



counterbalances the pressure of a mass of hydrogen contained in a 

 lateral vessel. The whole apparatus is sunk in the ground with 

 the exception of a short column of mercury at the top. The level 

 of the liquid is compared with a lateral division, the image of which is 

 formed in the axis of the tube, and the points may be fixed to 

 within the j-Jtj of a millimetre. 



Direct observations at different times of day only showed a 

 continuous course, the greater part of which was due to inevitable 

 changes of temperature ; certain results can ouly be obtained by 

 photographic registration. 



In the proofs submitted the differences of level are multiplied 

 by 20 ; they correspond to the variations which are directly ob- 

 served on a column 90 metres in length. 



The curves ordinarily present a very regular and slow course 

 which is especially due to changes of temperature : but for some 

 days sudden accidents are seen, the duration of which is from 

 fifteen minutes to an hour, and which do not seem to be explicable 

 otherwise than by correlated variations of gravity. These accidents 

 may attain and even exceed -^ of a millimetre, which corresponds 

 to -g-o^TTF or one secon( i per day, supposing that they lasted the 

 whole day. 



In order to have a term of comparison, it is sufficient to observe 

 that if the difference between high and low water is 10 metres, 

 the liquid layer would produce a variation of ttottWo" °^ ^he ^ eve ^ 

 value of gravity, that is one fifth of the preceding. 



The existence of these temporary variations of gravity appears 

 undoubted and deserves attention. I intend to organize at the 

 Observatory of the Pare Saint Maur an apparatus constructed 

 with more care, from which all casual trepidation of the ground is 

 excluded, and the indications of which can be continuously followed. 



Observations of this kind will no doubt present a peculiar interest- 

 in volcanic districts if the changes are due to displacements of 

 masses in the interior. — Comptes Rendus, January 30, 1893. 



PRELIMINARY NOTE ON THE COLOURS OF CLOUDY 

 CONDENSATION. BY C. BARUS. 



By allowing saturated steam to pass suddenly from a higher to 



a lower temperature (jet) in uniformly temperatured, uniformly 



dusty air, the following succession of colours is seen by transmitted 



white light, if the difference of temperature in question continually 



increases : — Faint green, faint blue, pale violet, pale violet-purple, 



>ale purple, muddy brown-orange, straw-yellow, greenish yellow ; 



;reen, blue-green, grey-blue, intense blue, indigo, intense dark 



riolet, black (opaque); intense brow r n, intense orange, yellow, 



diite. 



Seen by reflected white light, the same mass of steam is always 

 Lull neutral white. 

 If the colours enumerated be taken in the inverse order, be- 

 inning with white, they are absolutely identical with the inter- 

 ference-colours of thin plates (Newton's rings) of the first ami 



