418 Chronicles of Science. [July, 



passage of jMercury's centre across the sun's limb at ingress until 

 exterior contact at egress. Tlie result of the examination of twenty- 

 two observations is as follows : — 



Correction to excess of Suu'u E.A. over Mercury's = - 0""92 - -092 t; 

 N.P.D. „ = - 0"-16- -OiST; 



where r is the assumed error in the determination of the longitude 

 of the Capo of Good Hope Observatory. The observations were 

 made with the 8i-foot equatorial. Sir T. Maclear adds some 

 remarks on the observed diameter of Mercury. He deduces from 

 all the observations the value 8" • 376, with a probable error of 

 ± 0"*11. This result is less than the tabular diameter by I"' 50. 

 As the apparent diameter of Mercury is increased through the 

 effects of kradiation when the planet is observed off the disc of 

 the sun, and diminished through the same cause when the planet is 

 in transit, the difference is readily exjjlicable. 



The Astronomer Eoyal comments on M. Puiseux's statement that 

 Halley's method can be applied with advantage to the transit of 1874. 

 He interprets ]\I. Puiseux's carefuUy- worded statement to signify that 

 in the French mathematician's opinion Halley's method is better 

 than Delislo's. We do not find in M. Puiseux's paper any passage 

 which can be so understood. All that Puiseux has said is in one 

 place that the message can be applied " advantageously," and in 

 another that " there can be no reason why it should not be applied." 

 It would be to exaggerate the requirements of com-tesy in scientific 

 discussion to assume that ]\I. Puiseux had really another meaning, 

 but was prevented by courtesy from expressing it. Nor will his 

 figures bear such an interpretation. He points to statistics which 

 give intervals somewhat greater than those which come out by 

 Delisle's method ; but so skilful a mathematician could not but be 

 aware that Halley's mode involves four observations of contact, 

 Delisle's only two ; and that consequently more had to be consi- 

 dered in forming a comparison between their relative advantages 

 than the mere length of the intervals involved. The careful perusal 

 of M. Puiseux's pamphlet suggests the impression that what he 

 reaUy had in his mind in indicating a difference of opinion with the 

 Astronomer Eoyal was the latter's statement that Halley's method 

 " fails totally " in 1874. The concluding sentence of Mr. Airy's 

 note indicates a change of opinion on this point. After expressing 

 his wish that Halley's method may bo applied to the transit as well 

 as Delisle's, he adds, " Every series of observations which can really 

 be brought to bear upon this important determination will bo 

 valuable." 



Mr. Stone supjolies an interesting paper on a " personality " in 

 the determination of the line of colhmation of a transit instrument. 

 The existence of a personal error of this sort to an appreciable 



