51^ 



NA TURE 



[SePTEMEEK 2 2, 1904 



According to a brief report by J. Step, director of the 

 Joacliimsthal Mine, published in the Proceedings of the 

 \'ienna Academy of Sciences (No. 14), freshly excavated 

 uranium ore, which has never been exposed to the light, is 

 strongly radio-active. .A comparative study of the activity 

 of illuminated and uniiluminated specimens of the ore has 

 yet to be made. 



In vol. vii. of the Fortschritte auf dem Gebiele der 

 Rontgenstralilcn Dr. Josef Rosenthal discusses the relative 

 advantages of large and small induction coils for producing 

 X-rays. When the tube used is not too highly exhausted, 

 and consequently has not too great a resistance, a small 

 coil giving a comparatively short spark may be used with 

 good results. Small coils have, moreover, the advantage of 

 being more portable and less costly than large coils. But 

 when a tube with a high vacuum is used a higher tension 

 coil has to be employed, and in such cases, in order to 

 prevent the tube from changing during a long e.xposure, 

 the number of interruptions per second must be reduced as 

 much as possible. 



OUR ASTRONOMICAL COLUMN. 



The Retlirn of Encke'.s Comet (1904 h). — As announced 

 in these columns last week, Encke's comet was re-discovered 

 at Koenigstuhl-Heidelberg on September 11. 



It was found by Herr Kopff, who describes it as being, 

 at present, a faint object. According to a note by Mr. 

 Denning, however, mentioned in Nature for July 21, the 

 favourable conditions of 1805, 1838, and 1871 should be 

 repeated during the present apparition, and it is possible 

 that the comet may become visible to the naked eye when 

 near to Altair, early in December. 



The accompanying chart given below shows, approxi- 

 mately, the apparent path of the comet among the stars 

 from now until October 15, according to the daily 

 ephemeris published by MM. Kaminsky and Ocoulitsch in 

 No. 3962 of the Astroiwmischi: Nnchrichton • — 



\"ariations in the Lunar LANDScAri:. — .\ communicatiun 

 from Harvard reports that Prof. W. H. Pickering, at pre- 

 sent located at the Lowe Observatory, California, observed 

 a bright hazy object 2" in diameter upon the floor of the 

 lunar crater Plato on July 31. Six previous observations 

 made between July 21-28 inclusive gave no indication of 

 this novel feature. 



On August 2 a black elliptical shadow two miles in 

 diameter was seen in the place of the previously observed 

 bright spot, whilst to the north-east and north there 

 extended a large white area, the existence of w'hich was 

 confirmed by an observation made on .August 3. 



A telegram dated August 22 states that real conspicuous 

 changes have taken place in this region during the past 

 month, and confirms the existence of the new crater, which 



has a diameter of about three miles. The bright area has 

 shifted considerably since .August 3. 



Several other objects which have not been mapped before 

 were observed whilst examining Plato, and it was seen that 

 the previously conspicuous white area surrounding craterlet 

 No. 54 (Harvard College Observatory Annals, vol. xxxii., 

 plate X.) has now disappeared. 



Sun-spot Periodicity and Terrestrial Phenomena. — In 

 a brochure published at Rochechouart (1904), Prof. O'Reilly, 

 of Dublin, emphasises the important part which a know- 

 ledge of the periodicity of solar activity plays in the pre- 

 diction of terrestrial meteorological events, and also 

 demonstrates that the origins of several important historical 

 events may possibly be attributed to the meteorological, 

 effects of solar changes. 



After discussing the more recent droughts, such as have 

 caused distress in Australia and India, and showing that 

 these occurred at definite epochs of solar cycles, he shows- 

 that the successive floods which caused the formation of 

 the Zuyder Zee probably occurred at epochs of sun-spot 

 maxima. Similarly he points out that each of the ten 

 centuries in Etruscan chronology were approximately J22-2 

 (i.e. iiiixii, or nearly ii") years in length, that is to 

 say, they contained about eleven sun-spot periods, and he 

 supposes that the Etruscan era probably commenced from a 

 period of great cold, or maybe some mernorable flood, which 

 could be attributed to excessive solar activity. 



From a study of Briickner's sun-spot cycles. Prof. 

 O'Reilly believes that the year 1S95 was the culminating 

 year of a period of heat and drought, and that 1915 will be 

 the corresponding centre-vear of a period of cold and rain. 



Observations of the Recent Perseid Shower. — M. 

 Henri Perrotin, observing at Nice, saw 11S4 meteors, of 

 which 1041 were Perseids, during the nights of August 9-14 

 inclusive. The observations were made between the hours 

 of 8 p.m. and 3 a.m. each night at the meteorological 

 station of the Nice Observatory, situated at an altitude of 

 2740 metres on Mount Mounier. 



The Perseids, as shown in his tabulated results, were very 

 numerous, the maximum display of the shower occurring 

 on the night of August 11-12, especially between i a.m. and 

 3 a.m. The maximum for each night occurred between 

 midnight and 3 a.m. 



.\ notable feature of the display was that the meteors 

 appeared in groups of two or more, each group being 

 followed by a break five to fifteen minutes in length. 



The radiant of the shower was seen to be a fairly 

 extensive area, not a point, having its centre near to 

 7 Persei. 



The Perseids were white and very swift, whilst the paths ' 

 were comparatively short. On the other hand, the sporadic 

 meteors observed were of a reddish-yellow colour, their 

 paths were long, and they travelled slowly, leaving trails 

 which lasted for some seconds. 



These observations again emphasised the importance of 

 selecting a station situated at a high altitude where the 

 atmosphere is generally exceptionally clear (Comptes rendiis, 

 No. 9, 1904). 



RADIATION IN THE SOLAR SYSTEM.' 

 T PROPOSE to discuss this afternoon certain effects of the 

 energy which is continuously pouring out from the 

 sun on all sides with the speed of light, the energy which 

 we call sunlight when we enjoy the brilliance of a cloud- 

 less sky, which we call heat when we bask in its warmth, 

 the stream of radiation which supports all life on our 

 globe and is the source of all our energy. 



.As we all know, this ceaseless stream of energy is a 

 form of wave motion. If we pass a beam of sunlight, or 

 its equivalent, the beam from an electric arc, through a 

 prism, the disturbance is analysed into a spectrum of 

 colours, each colour of a different wave-length, the length 

 of wave changing as we go down the spectrum from, say, 

 1/30,000 inch in the red to 1/80,000 of an inch in the blue 

 or violet. 



But this visible spectrum is merely the part of the stream 

 of radiation which affects the eye. Beyond the violet are 



1 Afterr.oon address delivered at the C.imbrid»e meeting of ihe British 

 Association, .Xugust 23, by Prof. J. H. Poynting,''F.R.S. 



NO. I 82 I, VOL. 70] 



