There is another thing of scientific interest about occultations, though it has lost some of its importance in recent years. When such an event has been observed, the agreement of the predicted time with that actually recorded by the astronomer offers a most searching test of the correctness of our theory of lunar motion. We have already called attention to the great inherent difficulty of this theory. It is easy to see that by noting the exact instant of disappearance of a star at a place on the earth the latitude and longitude of which are known, we can both check our calculations and gather material for improving our theory. The same principle can be used also in the converse direction. Within the limits of precision imposed by the state of our knowledge of lunar theory, we can utilize occultations to help determine the position on the earth of places whose longitude is unknown. It is a very interesting bit of history that the first determination of the longitude of Washington was made by means of occultations, and that this early determination led to the founding of the United States Naval Observatory. On March 28, 1810, Mr. Pitkin, of Connecticut, reported to the House of Representatives on "laying a foundation for the establishment of a first meridian for the United States, by which a further dependence on Great Britain or any other foreign nation for such meridian may be entirely removed." This report was the result of a memorial presented by one William Lambert, who had deduced the longitude of the Capitol from an occultation observed October 20, 1804. Various proceedings were had in Congress and in committee, until at last, in 1821, Lambert was appointed "to make astronomical observations by lunar occultations of fixed stars, solar eclipses, or any approved method adapted to ascertain the longitude of the Capitol from Greenwich." Lambert's reports were made in 1822 and 1823, but ten years passed before the establishment of a formal Naval Observatory under Goldsborough, Wilkes, and Gilliss. But to Lambert belongs the honor of having marked out the first fundamental official meridian of longitude in the United States. MOUNTING GREAT TELESCOPES There are many interesting practical things about an astronomical observatory with which the public seldom has an opportunity to become acquainted. Among these, perhaps, the details connected with setting up a great telescope take first rank. The writer happened to be present at the Cape of Good Hope Observatory when the photographic equatorial telescope was being mounted, and the operation of putting it in position may be taken as typical of similar processes elsewhere. (See also p. 86.) [Illustration: Forty-Inch Telescope, Yerkes Observatory, University of Chicago.]