principles of their construction are complicated, and too obscure for the understanding of ordinary men; and they leave the Observatory without having derived from it any clear idea whatever.
In both cases, however, the difficulties are very much over-estimated: or rather, difficulties are assumed, which, in reality, do not exist at all. The measure of the Moon's distance involves no principle more abstruse than the measure of the distance of a tree on the opposite bank of a river. The principles of construction of the best Astronomical instruments are as simple and as closely referred to matters of common school-education and familiar experience, as are those of the common globes, the steam-engine, or the turning-lathe; the details are usually less complicated.
In the application of the ordinary principles of geometry and trigonometry to such Astronomical measures as those to which I have alluded, it may sometimes be expedient to resolve the process into several successive steps, and these steps may perhaps require different kinds of treatment. But the remark which I have made applies to every individual step; all are simple and within ordinary comprehension, and the only complexity arises from the circumstance that the student may find it necessary to have a clear view of several such steps at once, in order to perceive the connexion between the first standard of length and the numerical measure last obtained. With these impressions on my mind, I had long wished for some opportunity of endeavouring to explain to intelligent persons the principles on which the instruments of an Observatory are constructed, (omitting all details, so far as they are
