CORRESPONDENCE. N.B.-We do not hold ourselves answerable for any opinions expressed by our correspondents. To all communications must be annexed the name and address of the sender, as a guarantee of good faith. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. TWILIGHT. Sir,-Two years ago when I made some naked eye observations on Coggia's Comet (see Register for 1874, p. 220), I was somewhat puzzled by what seemed the excessive amount of twilight for the time of year. The almanacs all state that in latitude of Greenwich there is "real night' after July 21st, which was the last night on which the comet was visible here. As the difference in latitude (55 min. 42 sec.) between Greenwich and my house did not appear to be sufficient to account for it, I proposed to have made some observations on the duration of twilight last year, but moonlight prevented it-this year, however, absence of moonlight and settled weather has enabled me to attend to it, as follows:: July 20th, 1876, 12h. 12m. G.M.T. Atmosphere extremely transparent, a strong clear twilight-5 mag. stars (26 and 27 Lyncis), visible in N. nearly to the horizon. Sun's centre below horizon 17° 8′ (neglecting refraction). July 24th, 12h. 12m. Sun below horizon 17° 56'; sky tolerably clear, but atmosphere not very transparent. A decided twilight, but not so strong as on the 20th. July 25th, 12h. 12m. (280ft. above sea level). streaks of cloud shewed visible. Sun below horizon 18° 10'; barometer 29.80 Twilight similar to last night, a few horizontal sharply against the light; 21 and 22 Lyncis July 29th, 12h. 12m. Sun below horizon 19° 5'; barometer 29.70; atmosphere extremely clear and transparent. A very decided twilight, some elms stood out sharp and black against the light. 35 and 22 Lyncis visible. July 31st, 12h. 12m. Sun below horizon 19° 35'; barometer 29.52; atmosphere wonderfully transparent; a decided twilight in N. horizon, but small in amount. 31 Lyncis visible. After the 31st observations were stopped by the moonlight. It will thus be seen that the statement in most astronomical works that there is "real night" when the sun is more than 18° below the horizon, is not strictly correct. Gosford Green, Coventry. Lat. 52° 24' 20" N. Yours truly, WILLIAM ANDREWS. Long 1° 29′ 40′′ W. METEOR. Dear Sir,-On the 16th of this month while taking a transit observation on Venus, and whilst watching for the ingress of the planet in the field of my small alt azimuth, by Jones, I observed a brilliant meteor passing through the field of the telescope, which the enclosed diagram will best explain The meteor presented itself as a perfectly round disc, brilliantly white and of the size of a star of first-rate magnitude. Its descent lasted about two or three seconds ere I lost sight of it in the field of view, and the time of my regulator, when at once I looked round, showed 21 minutes before 12 o'clock, mean Greenwich time. I wish also to mention that instead of a diagonal eye-piece I have adapted the prism of the camera lucida from my microscope to the eye-piece of my transit instrument, and which acts most excellently. I remain, dear sir, 8, Montpellier Terrace, Brighton: 27th July, 1876. Yours very respectfully, F. M. D'ALQUEN. P.S.-I wish, with your kind permission, to supplement a few remarks concerning my employment of a camera lucida instead of my diagonal eye-piece. The ordinary eye-piece of my transit instrument has a small projecting brass rim, to which I adapted the camera lucida. The advantage I derive from this arrangement I find is, firstly-that I get a more perfect image of the object under observation: secondly-that when I get stars of lower altitudes I have merely to remove the camera, . and use direct vision, thus obviating, as I sit before the instrument, any change of position, and so particularly avoiding the very objectionable stooping down one, and thirdly-having not to alter the focus of the telescope. As any little practical hint, which may add to the better observation and comfort of the observer, may be of service to other amateurs in possession of small transit instruments like myself, I felt induced to write these few lines for insertion, if you should consider them worthy enough. AUGUST METEORS. Dear Sir,-Last night I saw sixty meteors during a watch extending from 9 to 12. A large number had trains, and though the quantity was smaller than sometimes, the quality was very good. About 11.24 a very fine one appeared about 6° north, preceding a Coronæ Borealis, passing between that star and Boötis, and died out a few degrees S. W. of a Coronæ. It was much brighter than Venus, appearing as a broad pale green flash, leaving an orange train for fully a minute, so I had time to note its position fairly well. If anyone wishes for a map of it, to compare with other observations, which will most probably be forthcoming, I shall be very glad to send it. About twenty out of the sixty were obviously not "Perseids," some radiating from Cygnus. Yours truly, Writtle, near Chelmsford : August 12th, 1876. H. CORDER. 230 Dear Sir.-The perusal of M. Guillemin's Le Ciel, far inferior in solid value to Mädler's Popul. Astron., Berlin, 1841, induced me to form the annexed Tab. of sun's meridian altitude at various geograph. latitudes of Venus, supposing the equator inclined at seventy deg. (not 23 as with us) to her orbit, and referring the quarter year of 57 of her days to our year of 91 of our days, so as to familiarize us with the diurnal change of her climates. Formula sin. decl. = sin. long. (81° 33' p. day) × sin. 70° (log. 9'9729858). I assume one Venus' day of our calendar ones. Alts. below bar - are our "sub polo." When sun's decl. s. exceeds co-lat. N. he will not rise in N. lat., therefore invisible for interval indicated in tab. under each lat. Sun repasses zenith on the days indicated in tab. Climates seem to resemble that of St. Petersburg, Norway, &c., except near equator. This assimilation to our vulgar percentage of northern half-year, will more strikingly exhibit the climates, the hybernating habits of the fauna, peculiar flora, &c., as I am strongly possessed with the idea that our earth contains living and generating specimens of organisations under every variety of habitable data, and the farther planets may have beings like our deep water fish, our moles, our proteus of the Adelsberg caves in their corporeal adaptations. For 70°, 72° or 75° give climates more variable. Further, I noticed thirty years ago that our recent binary star astronomy is a primitive feature of the astronomy of Venus or Mars: for the greatest elongation angle of our moon from our earth is to them 240000 and 240000 3438' (9468) 106 Venus and 3438' to 982 204 or 3 and 17' for Mars, which elongation reduce to zero in 7 of either planet's day, and this possibly was the ground of an archaic calendar, such as terrestrials possess in the lunisolar periods: thus 99 times 29 53 days equals 8 × 365 or 13 × 2249 serves for fundamental Venus calendar. Guillemin makes express train 80° L. 70° L. 60° L. 50° L. 40° L. 30° L. 20° L. 10° L. 0° 22 Sp. 3 Oct. 13 Oct. Ju. 22 Jy. 16 Jy. 28 Ag. 7 Ag. 16 Ag. 27 25 Oc. 7 No. 17 No. 30 No. 14 Ja. 11 Ja. Dc. 22 always visible by day. Sp. 9 Sp. 22. THE MOON. Sir,-In your review of my work on the moon in the last number of the Astronomical Register, it stated that no information is given with regard to the power with which the different tinted drawings of the various lunar formations were made. Reference to the right hand corner of each drawing will show, however, the power employed, which was 330 for the drawing of sunrise on Agrippa, 450 for the same under meridional illumination, the same for Plato near sunrise, and 530 for Copernicus, and 600 for Plato near full moon. I should not have referred to the above point had it not been that I should like to draw the attention of lunar observers to the question of the best powers for lunar observation, and in particular to the study of the aspect of lunar formations under meridional illumination. For ordinary observation of the surface of the moon when under oblique illumination, eye-pieces of a magnifying power of from 45 to 35 to the inch, according as the aperture varies between 4 to 10 inches, will in general be the best. This is a circumstance I believe generally familiar to all observers of our satellite, for a little experience soon shows that higher powers render the detection of minute dark shadows and other features a task of considerable difficulty. When a region is thoroughly familiar, and the definition of the highest excellence, somewhat higher powers can be occasionally used with advantage, but it will, I think, be rarely found that with aperture of from 8 to 10 inches a higher power than 450 can be used with advantage. Smaller telescopes will permit proportionately higher powers in accordance with a well understood fact. What however is, I believe, not so well known a circumstance is that when observing a lunar formation under high illumination, as high a power as the state of the atmosphere will permit will often prove in the highest degree serviceable. If the definition of the telescope is of sufficient excellence, and the state of the atmosphere does not prohibit its use, for the study of a region of the moon under high illumination, a power of from 70 to 80 to the inch will generally be found the best that can be employed. Under these circumstances it is surprising the great amount of lunar detail which can be made out; details which under oblique illumination are hidden in the deep shadows, can often be detailed with facility. On the other hand, a moderate power, such as would be most advantageous under ordinary circumstances, will usually fail to show any detail whatever. The rationality of this is obvious, the high power not only by diminishing the total amount of light, but also in increasing the apparent dimensions of the various delicate gradations of shade which mark the different angles of slope of the various formations, enables them to be seen with ease. Under these conditions, with 9-inch aperture and a power of about 750, the entire rill systems in connection with the great rills of Ariadæus and Hyginus can be seen as two delicate irregular white lines due to the rugged slopes of the rills, enclosing a delicate greyish line, the floor of the rills. The principal members of the rill systems of Triesnecker and Ramsden can also be seen at Full, generally as thin white lines, and occasionally these two lines can be seen divided by the extremely delicate darker line from the floor of the rill. In Full, the great brightened plateaux of the Apennines and Caucasus exhibit innumerable gradations of delicate greyish white, and enable these most difficult regions to be better mapped than at any other period. Whilst writing on this subject I may take the opportunity of explaining that in the work I have systematically followed Mädler's nomenclature |