ASTRONOMY. 13 



and photospheric areas were deduced from the Kew observa- 

 tions of spots; and fr6m a consideration of these data, and 

 confining the question strictly to changes of terrestrial tem- 

 perature due to this cause alope, Langley deduces the result 

 that " sun-spots do exercise a direct effect on terrestrial tem- 

 perature by decreasing the mean temperature of the earth 

 at their maximum." This change is, however, very small, 

 as "it is represented by a change in the mean temperature 

 of our globe in eleven years not greater than 0.3 C, and not 

 less than 0.05 C." 



Professor Langley has also had constructed an apparatus, 

 which is similar in principle to one devised by Dr. Hastings. 

 It consists of two prisms by which two spectra from various 

 parts of the sun can be juxtaposed. Comparing the light 

 from the two limbs, for example, one is enabled to discrim- 

 inate the atmospheric lines, and the proof of solar rotation 

 and a measure of its velocity is easily to be had. It has also 

 other applications. 



Nyren, of Pulkova, has published an important paper on 

 the position of the equinox for 1865.0, derived from obser- 

 vations of the sun made with the Pulkova transit instrument 

 (Wagner) and vertical circle (Dollen and Gylden). The de- 

 duced position of the equinox differs by +0.064" from that 

 assigned by Greenwich observations, by +0.055" from Pul- 

 kova (1845), by +0.011" from Paris, and 0.002" from Wash- 

 ington. 



Professor Wolf, of Zurich, who has collected all available 

 data in regard to sun-spots for nearly twenty-five years, has 

 now these data for more than 22,000 days between 1749 and 

 1876. Since 1848 these data may be said to be complete. 

 The mean value of the solar-spot period he finds to be 11.111 

 years 0.307 ; but single periods may be two years longer 

 or shorter than the mean. The maximum is nearer the pre- 

 ceding minimum than the following one. A longer period 

 of about 178 years (not 55) is also indicated. It is to be 

 noted that 16 sun-spot periods, 15 revolutions of Jupiter, 6 

 of Saturn, and 298 of Venus are nearly equal. 



Sun-spots continue to be observed photographically at 

 Greenwich, Paris, Moscow, Toulouse, Kasan,Vassar College, 

 and are observed visually at Madrid, Oxford, Berlin, Zu- 

 rich, Leipzig. 



