INTRODUCTION.
In conversing with persons who are not officially attached to Observatories, or in other ways professionally cognizant of the technicalities of practical Astronomy, but who, nevertheless, display great interest in the science of Astronomy generally, I have frequently been struck with two remarks. The first is, that these persons appear to regard the determination of measures, like those of the distance of the Sun and Moon, as mysteries beyond ordinary comprehension, based perhaps, upon principles which it is impossible to present to common minds with the smallest probability that they will be understood; if they accept these measures at all, they adopt them only upon loose personal credit; in any case, the impression which the statement makes on the mind is very different from that created by a record of the distance in miles between two towns, or of the number of acres in a field. The second remark is, that when persons well acquainted with the general facts of Astronomy are introduced into an Observatory, they are, for the most part, utterly unable to understand anything which they see; they are impressed perhaps with the apparent complexity of subsidiary parts of the Astronomical instruments, and they imagine that the fundamental
