1868.] Astronomy. 379 
Mr. Key considers that there is a progressive character in the 
results above recorded, since the various aspects of the nebula do 
not appear to depend on the power employed. His own reflector 
is somewhat more powerful than the Herschelian 20-feet reflector, 
but is of incomparably inferior power to the instruments of Lord 
Rosse and Mr, Lassell. “‘ One fact,” he adds, “seems, at all events, 
abundantly evident, viz. that whereas at the date of the Herschels’ 
observation, there was no appearance whatever of a ring surround-. 
ing the central star, at the present time there are two.” 
Mr. Simms supplies a description of a zenith telescope em- 
ployed in America, and explains a method of determining the 
latitude which has been for several years adopted by the United 
States coast surveyors, and which has the advantage of being at 
once simple and exceedingly accurate. The instrument is further 
described, and figured in a later number of the Society’s notices, in 
a paper by Mr. Davidson, of Germantown, Pennsylvania. ‘The 
method of observation, known as Talcott’s, is worth studying :-— 
“Two stars are selected, one of which passes the meridian to the 
north, and the other at nearly the same distance to the south of the 
zenith, The telescope is brought into the plane of the meridian, 
and set for the star which first passes the meridian ; when visible it 
is bisected by the micrometer wire, the tangent screw of the instru- 
ment being used. ‘The telescope is then turned 180° in azimuth, 
and when the second star makes its appearance, should there be any 
difference in the zenith distances, this distance is measured by the 
micrometer screw.” ‘The latitude is readily deduced. For example, 
suppose the polar distances of the two stars are D and D’ (D less 
than D’), and that the former star, when on the meridian, has a zenith 
distance less than the corresponding zenith distance of the other by 
a small are d; then a star, whose polar distance was D—d, would 
eross the meridian at exactly the same altitude towards the north, 
as the star whose polar distance is D’ has towards the south. Hence 
the polar distance of the zenith of the place of observation, that is, 
the complement of the latitude, is } (D—d\+ D'). Mr. Davidson 
says that, after twenty-two years’ experience in using prime vertical 
transits, vertical circles, and Airy’s zenith sector, he can affirm con- 
fidently that the zenith sector of the coast survey, used as above 
described, is far better than any of them. 
Father Secchi supplies an interesting paper on the great nebula 
in Orion. He has also sent to the Astronomical Society a drawing 
of the nebula, commenced several years ago, and finished last year 
by combined observations made by himself and Father Ferrani, his 
assistant. One point in Father Secchi’s paper will excite surprise. 
He states that the nebula is mtich better seen in moonlight than on 
dark nights. He considers that this is a consequence of that 
optical principle, that the difference of two lights is more easily 
