90 Chronicles of Science. [Ji 



an. 



using both the wire-micrometer and the double-image micrometer 

 in delicate measurements, obtains results appreciably coincident ; so 

 that, as Professor Kaiser remarks, " the discrepancies are far more 

 to be sought in the observers than in the instruments." Keferring 

 specially to double-star measurements, he remarks further, that 

 they appear " far too inaccurate for the consequences one will 

 derive from them." The Professor's list of observations serves to 

 exhibit the close approach to coincidence attained by the use of the 

 two instruments, and to afford to observers new estimates of some 

 favourite test-objects. 



General Shortrede, discusses the effect of the vapour of mercury 

 in depressing the ths&kometric column. In temperate climates 

 this effect is not often appreciable, and except in very delicate 

 experiments may perhaps be safely disregarded ; but in the tropics, 

 or in exceptionally warm weather, the height of the mercurial 

 column is very sensibly depressed from this cause. In India, for 

 instance, General Shortrede found that the true reading, obtained 

 after tilting the barometer so as to condense the vapour, differed 

 from the observed reading before that operation by from 10 to 

 20-thousandths of an inch, and on one occasion by so much as 

 •023. The tubes were in exceptionally good order, one having been 

 boiled more than twenty times, the vacuum being so perfect that 

 after the tube had been placed some hours in a horizontal position 

 " the mercury, by electrical attraction, would adhere to the top of 

 the tube, and not separate till shaken by tapping," — the tube of 

 32 in. remaining full in this way, at Pana, where the average 

 height is 28 in. ! 



The subject seems worthy of investigation, since if we would 

 learn the laws regulating the variations of atmospheric pressure, 

 the minutest circumstances affecting the truth of barometric indi- 

 cations must be recognized, so as to be either eliminated or cor- 

 rected. The Greenwich photographic registrations are evidently 

 liable to be peculiarly affected by a cause of this kind. General 

 Shortrede noticed, indeed, that on one warm day of the past summer 

 the vacuum of the barometer for outside indications (at Greenwich) 

 was studded with minute globules of mercury, derived from the 

 condensation of the mercurial vapour. 



Father Secchi sends a drawing of the spectrum of Antares (the 

 Sirius of red stars). Antares attains a sufficient elevation in the 

 latitude of Borne for satisfactory observation with the spectrometer. 

 As might be expected the spectrum (which, by the way, is presented 

 in a reversed position) exhibits a crowding of lines towards the more 

 refrangible (or violet) end, and several spaces clear of lines, or in 

 which lines are more sparsely strewn, towards the red end. 



Captain Noble remarks that Jupiter's third satellite reappeared 

 two minutes before the time predicted in the Nautical Almanac. 



