KNOWI.KDGi:. 



Jamtaky. 1912. 



TIh' following is :i siiininary of oiir it'siiils: 



From 14° JO' S to 4 20* S. coiiliiiiioiis dawn. 

 l"iom 4° 20' S to 2 40' N. altoiiiatioiis of twilij;lu ;iikI 

 SMnlight. 



l"ri>Mi 2" 40' N until llip Sun cnniplctcs his most Northcrii 

 course and returns again to 2 40' N, llic-ro will be no sunset, 

 but one long d.'iy. 



Wf shall now refer In the Niuitic.il .Ahnanac for this year. 

 On I'ehruary lOlh. about ') p.m.. the Siui attains a declination 



FlGlKK .56. 



14° JO' S. Tlic continuous dawn has then commenced. On 

 M.uch llth. at noon, his dechnation is 4° 2' 24" S., so that 

 twilight has commenced some tiiTie previous to this. The 

 interval is about 2SA days. If. however, we take 17" as the 

 aiiijlf below the horizon when twilight starts, we should recliou 

 from IJ' 30' S. This would reduce the time of continuous 

 dawn to 25i days. It is easily seen from this how impossible 

 it is to give the time of continuous dawn exactly, because 

 the angle below the horizon varies with circumstances, 

 such as atmospheric conditions, and so on. It is not exactly 

 18° under all circumstances. 



Or. March 28th, at noon, the Sun is just short of 2° 40' \. 

 by 30" • 7. Practically, then, we may say that the long day has 

 commenced on March 2Kth. Reckoning from March llth to 

 March 2Sth. we obtain about 17 days as the time of alternation 

 of sunshine and dawn. 



On September 17th, about 8 a.m., the Sun has again attained 

 a declination of 2" 40' N. The time from March 2Sth to 

 September 17th is 173 days. But we umst deduct slightly 

 from this, because we have assumed daylight to start the 

 instant the upper edge of the Sun appears above the horizon. 

 We must allow a little time after this before the long day 

 actually commences. 



l-'or the reasons just given, we can only obtain an approxi- 

 mation to the number of days in each period. The figures 

 may vary slightly according to local conditions. If we adhere 

 strictly to the above reasoning, a latitude 86^ 8' \. gives 

 171 days for the long day. Probably 8f).J° N. most nearly 

 fulfills all the conditions, because a less latitude than 86}° N, 

 increases the time of alternation of sunshine and twilight. 



It may be interesting to notice that in Lieutenant Shackle- 

 ton's book, "The Heart of the .Antarctic," it is mentioned how 

 the Sun was seen for the last time in I'JOS, on .April 27th, 

 when one-third of the disc was above the horizon at noon. 

 He was seen again first on August 17th, when his entire disc 

 was above the horizon, and the lower edge one- fourth his 

 diameter from it. He could, it is stated, have been seen a 

 day or two earlier if the day had been clearer. The Latitude 

 was 77 30' S. This fact shows how difiicult it is to predict 



the above times accurately to a day, as fogs and ini.sts will 

 always modify the results. 



The Sun cannot appe.ir in the manner suggested. Con- 

 sidering that his declination is increasing by about 23' 30" at 

 that time, and that his apparent diameter is i2'. he could not 

 take more than two days to rise above the horizon. If. in 

 ecpiation (II. we substitute 4' 2' 24" for «, the declination on 

 March llth, and 90° 50' for 7., and solve for h. we find 



h 23 42', .'. „ =1-58 hours. The total length of the day is 



greater than twice this, since the increasing declination makes 

 I he afternoons longer than the mornings. His upper edge is 

 above the horizon for a longer time than 3 hours 9 minutes on 

 this d.iy. 



l-'or the next day we find the time to be 4 hours 50 minutes- 

 and for the next 6 hours 8 minutes. The whole number may 

 be easily worked out from the formula. In these examples we 

 assume 5 to be that of noon, and neglect its small change in 

 the time. Of course these times vary from year to year, as 

 the Sun will not attain to precisely the same declination on the 

 same day each year. The first day would be 1' hours long, if 

 the Sun's declination were somewhat less than 4° 2' 24". 



If we reckon the time from which the Sun actually attains 

 4 20' S., that is, on March 10th at 6 p.m.. to the time when 

 he enters the Vernal Equinox, that is, 6 p.m. on March 21st, 

 the interval is 1 1 days. From a few calculations we find the 

 change of longitude in the time to be 10 55'. If this time on 

 March 10th correspond with the instant the Sun is in the 

 constellation mentioned. 120°. then at X'ernal Fquinox we 

 shall have 129°+10 55' = 139°55'. diflering by 3' from the 

 longitude given. This difference corresponds with a time 

 of about 1 hour 12 minutes, so that the calculation by " Baroda" 

 is practically correct for this period. 



If some liglit were thrown on the precise time these 

 phenomena occurred, it would simplify the c.ilculations asked 

 for with reference to New Moon, and so on. 



(Rev.) M. Davidson. 



51. (".KOL(X;V OF SOUTH DORSET (ISLE OF 

 FL'RBECK). — The geology of this district offers wide scope 

 for practical field geology on account of the number of strata 

 which are so well exposed in coast sections. A detailed 

 account as suggested by S.P.R. would necessitate quite a 

 lengtliy article to deal at all adequately witli the geology from 

 the point of view of strata alone, without mentioning fossils 

 and minerals. 



The chief places of interest in the locaHty are as follows : — 



(1) Durlston Bay, where a splendid section of the Purbeck 



beds is exposed, and in fact duplicated in part by a 

 fault. 



(2) Swanage Bay. with Upper Purbeck at Peveril Point. 



Wealden. Lower ("ireensaud. I'pper Greensand and 

 Clialk at the more northerly end of the bay. 



(3) Studland Ba>'. with I'ppcr Chalk and Bagshot sands. 



(4) St. Alban's Head and Chapman's Pool, with Portlaudian. 



Purbeck and Kinnneridge Clay. 



(5) Kimmoridge B.iy. where many faults almost diagranunatic 



in their distinctness may be seen. 

 (61 Lulworth Cove and Stairhole. showing steps in Cove 

 formation and strata compressed and contorted. 



(7) Durdle Cove, showing further stages of coast erosion 



and cove formation. 



(8) Bournemouth, with Bagshot and Bracklesham Beds 



containing noted leaf beds. 



I would suggest that S.P.R. should purchase "The Guide to 

 the Geological Model of the Isle of Piubeck," by .Aubrey 

 Strahan, M.A.. F.R.S. ; Price Od. Publishers: E.Stanford, 

 12. 13 and 14. Long .-Vcre, London, which gives an interesting 

 and useful account of the district required. 



Hekuekt 1-. Tavlok. 



