Page:In the high heavens.djvu/171

 chart of the heavens. About 22,000 photographs will be necessary, each covering a space of four degrees. Each star is to appear on two plates, so as to avoid errors, and by giving an exposure of rather less than an hour it is expected that all stars down to the fourteenth magnitude will be represented. Astronomers well know how large a share of credit is due to Sir David Gill in connection with this great work. This vast surveying task is only one of the pieces of astronomical work in which photographing the stars is now employed. In the delicate movements of annual parallax it has been proved that measurements made on the photographs can compare favourably with the finest measurements made on the heavens. We are only just beginning to realise the benefits from these photographic processes.

Sir William Huggins referred to the constitution of comets and their connection with meteors. Nothing is better established than the fact that the periodic meteor shower is a swarm of minute bodies revolving around the sun in an elliptic orbit, and that in the case of some of the greater showers, at all events, the highway pursued by the meteoric shoal is also the highway in which a great comet moves. That there is a connection between comets and meteors of this periodic class seems therefore unquestionable, though it does not seem easy to say what the precise nature of the relation may be. It is, however, especially necessary to observe the distinction between the ordinary luminous meteors and the solid meteorites which occasionally tumble down on the earth. It does not seem to be at all clear that meteorites have any connection whatever with comets. The meteorites do not stand in any ascertained relation