July 5, 1900] 



NA TURE 



235 



■ 



At the date of the last report the measurement of the plates 

 LS completed from December 64° to 69° ; and in Zone 70° 

 im R. A. oh. to I3h. 48m. During this year Zone 70° has been 

 ished and Zones 71° and 72° have been measured, with the 

 eption of thirty-six quarter plates. Subject to this exception, 

 measurement is complete from December 64° to 73°. 

 Good progress has been made with the printing of the 



measures. Zone 64" is finished and Zone 65' as far as R.A. 



2ih. 36m. It is estimated that all the measures from December 



64° to December 72" will be included in one volume of about 



650 pages. 



HeLIOGRAPHIC OB.SERVATIONS. 



In the year ending 1900 May 10, photographs of the sun 

 have been taken on 180 days, either with the Thompson or 

 Dallmeyer photo heliographs. The former, mounted on the 

 Thompson 26-inch refractor, was used as the regular instrument 

 for solar photography up to March 9, when it was temporarily 

 disrnounted, the Dallmeyer photo-heliograph being substituted 

 for it. Of the photographs taken with either instrument, 369 

 have been selected for preservation, besides 11 photographs 

 with double images of the sun, for determination of zero of 

 position angle. Photographs to supplement the Greenwich 

 series have been received from India or Mauritius up to 1900 

 March 8. 



For the year 1899, Greenwich photographs have been selected 

 for measurement on 202 day?, and photographs from India and 

 Mauritius (filling up gaps in the series) on 162 days, making a 

 total of 364 days out of 365 on which photographs are at pre.sent 

 available. 



The chief characteristic of the sun's surface, during the period 

 covered by this report, has been the steady decline in the mean 

 daily number and area of spots observed, August and September 

 1899 in particular showing a marked sub-minimum. 



Magnetic Observations. 



The variations of magnetic declination, horizontal force, and 

 vertical force, and of earth currents have been registered photo- 

 graphically, and accompanying eye observations of absolute 

 declination, horizontal force and dip have been made as in 

 former years. 



The regular observations of magnetic declination have been 

 made since 1899 January i, in the Magnetic Pavilion, alternat- 

 ing with determinations in the Magnet House (for effect of the 

 iron in the Observatory buildings), the observations in the Mag- 

 netic Pavilion being made with a hollow cylindrical magnet 

 mounted in conjunction with the large theodolite. 



The determinations of horizontal force and dip have been 

 made with the Gibson deflexion instrument and the Airy dip 

 circle mounted in the new Magnetic Pavilion, since 1898 

 September. 



The principal results for the magnetic elements for 1899 are 

 ..i follows : — 



Mean declination ... 

 Mean horizontal force 



16° 34' -2 West. 



r 3 '9947 (in British units). 

 \ 1-8419 (in Metric units). 

 Mean dip (with 3-inch needles) 67° 10' 13". 



These results depend on observations made in the new Mag- 

 netic Pavilion, and are free from any disturbing effect of iron. 

 The correction to the declination, as found in the Magnet House, 

 is - 10' 7, as deduced from the observations made with the new 

 declinometer in the Magnetic Pavilion. 



The magnetic disturbances in 1899 have been few in number. 

 There were no days of great magnetic disturbance and sixteen 

 ')f lesser disturbance. Tracings of the photographic curves for 

 these days, selected in concert with M. Mascart, will be pub- 

 lished in the annual volume as usual. The calculation of diurnal 

 inequalities from five typical quiet days in each month has been 

 continued. 



The question of the regulations to be enforced for the pro- 



ction of the Observatory from disturbance of the magnetic 

 leisters by electric railways or tramways in the neighbourhood 



now under the consideration of the Bjard of Trade. 



Meteorological Observation.s. 

 Consequent on the changes in connection with the new 

 Mservatory buildings, the shed containing the photographic 

 iiermometers was moved 15 feet towards the west on May 16 

 and 17, 1899. 

 The Kew Committee of the Royal Society has suggested that 

 NO. 1 60 1, VOL. 62] 



steps should be taken to assimilate the methods of registration 

 of atmospheric electricity with the Thomson electrometers at 

 Greenwich and Kew, and the question of the modifications to 

 be introduced into the Greenwich electrometer is now under 

 consideration. 



The mean temperature for the year 1899 was 50^7, being 

 I°'2 above the average for the fifty years, 1841-90. 



During the twelve months ending 1900 April 30, the highest 

 temperature in the shade (recorded on the open stand in the 

 Magnetic Pavilion enclosure) was 90'"0, on August 15. The 

 highest temperature recorded in the Stevenson screen in the 

 Observatory grounds was 88° '8 on the same day. 



The month of August was exceptionally warm, the mean tem- 

 perature for the month being 65°'5, which is 3^*9 above the fifty 

 years' average (1841-1890). This high temperature for the 

 month has only been reached before on one occasion in the 

 previous fifty-eight years, viz. in August 1857. The month of 

 November was also exceptionally warm, the mean temperature 

 for the month being 4° 8 above the average. 



The lowest temperature of the air recorded in the year was 

 18° "O, on February 9. There were fifty days during the winter on 

 which the temperature fell below 32°, a number slightly below 

 the average. 



The mean daily horizontal movement of the air in the twelve 

 months ending 1900 April 30 was 268 miles, which is 13 miles 

 below the average for the preceding thirty-two years. The greatest 

 recorded daily movement was 776 miles on April 13, and the 

 least 50 miles on October 22. The greatest recorded pressure 

 of the wind was 27 lbs. on the square foot, on November 3, and 

 the greatest hourly velocity 48 miles, on April 13. 



The number of hours of bright sunshine recorded during the 

 twelve months ending 1900 April 30, by the Campbell-Stokes 

 instrument, was 1636 out of the 4454 hours during which the 

 sun was above the horizon, so that the mean proportion of 

 sunshine for the year was 0367, constant sunshine being 

 represented by i. 



The rainfall for the year ending 1900 April 30 was 21*97 

 inches, being 2*57 inches less than the average of fifty years. 

 The number of rainy days was 146. The rainfall in the month 

 of August was only 0*354 inch, being the smallest August rain- 

 fall on record in the fifty-nine years, 1841-99. The next smallest 

 value was 045 inch, in August 1849. The rainfall in February 

 amounted to 3-58 inches, being the largest February rainfall on 

 record in the sixty years, 1 841 -1900, with the exception of the 

 February rainfalls in 1866 and 1879, which amounted to 4*03 

 and 3 81 inches respectively. 



The remaining portion of the report is devoted to the progress 

 in the printing and distribution of the publications and 

 chronometers, time-signals, longitude operations. (S:c. 



In view of the large additions to and modifications in the 

 instruments and buildings of the Royal Observatory in recent 

 years, it is proposed to prepare a full description of the Obser- 

 vatory, illustrated by photographs. 



It may be mentioned that the Observatory equipped and sent 

 out an expedition to observe the total solar eclipse of May 28, 

 having received the sanction of the Admiralty. The Astronomer 

 Royal, with Mr. Dyson and Mr. Davidson, left for Ovar, in 

 Portugal, on May 11, taking with them the Thompson 9-inch 

 photographic telescope, the new 4-inch enlarging lens for large- 

 scale photographs of the corona, a pair of photographic spectro- 

 scopes with heliostat, lent by Captain Hills, for photographing 

 the spectrum of the lower chromosphere and of the corona, and 

 a double camera, on one of the photo-heliograph mountings, with 

 lenses of 4 inches and 2.\ inches aperture for photographing 

 the coronal streamers. 



An examination of the fine photographs that were obtained by 

 the party, which were shown on the day of the visitation, gave 

 one a good idea of the success which had rewarded their efforts. 



THE GEOLOGICAL AGE OF THE EARTH.^ 

 "Y^HILE, in his efforts to arrive at an estimate of geological 

 time, the geologist himself is seriously hampered by the 

 uncertainty of the data at his disposal, he has followed with ex- 

 pectant interest the successive attempts made by votaries of 



1 "An Fstimate of the Geological Age of the Earth." By J. Joly 

 M.A., D.Sc, F.R.S., Hon. Sec. Royal Dublin Society ; Professor of 

 Geology and Mineralogy in the Univer«it^ of Dublin. Pp. 44. (Scu-nti_fic 

 Transactions of the Royal Dublin Society, vol. vii. Ser. ii. Dublin. 

 1899.) 



