THE AMERICAN JOURNAL OF SCIENCE [THIRD SERIES.] ་ ART. XXXIII.—On the Deflection of the Plumb-line_and [Published by permission of the Superintendent of the U. S. Coast and Geodetic Survey.] ON the return of the Solar Eclipse Expedition from the South Seas, in 1883, two of the party stopped in the Sandwich Islands for the purpose of determining the force of gravity. An old pendulum station on the island of Maui, occupied by DeFreycinet in 1819, was the point chosen. While there the latitude of the place was determined with precision, by the method of equal zenith distances. This latitude was carried back by triangulation to Honolulu by Professor Alexander, the Surveyor-General of the islands. It was there compared with the local astronomical latitude as given by Captain Tupman, of the British Transit of Venus Expedition of 1874. Their comparison revealed a discrepancy,-quite incompatible with the accuracy of the work, as shown either by the star observations at the terminal points, or by the triangulation connecting them. This seemed to prove, as indeed had long been suspected by Professor Alexander, that there were unusual plumb-line deflections. Both the excess of matter in the high mountain masses above, and the defect of matter in the deep sea below, would indicate this. AM. JOUR. SCI.-THIRD SERIES, VOL. XXXVI, No. 215.-Nov., 1888. In view of the importance of the determination of these deflections in the survey of the islands, as well as the general scientific interest in the subject, the Hawaiian government was led to ask the cooperation of the United States in the work of determining with precision a number of astronomical latitudes. Accordingly, in December, 1886, with the sanction of the Honorable Secretary of the Treasury, the Superintendent of the Coast and Geodetic Survey loaned the necessary instruments, and granted one of the members of the service a leave of absence long enough to make the observations On the part of the Hawaiian Survey, Messrs. F. S. Dodge and W. A. Wall assisted in the gravity observations. Mr. Wall was permanently attached to the party and recorded all the latitude work. The credit of the work, therefore, belongs in part to both governments. The Island Survey bore all the expenses, and selected the stations; the Coast and Geodetic Survey loaned the instruments, furnished the observer, and is making the computations. Later it was thought desirable to supplement the latitude work, by the determination of the force of gravity on the top of one of the mountains, and at the sea-level. This would give a check on the deflections determined at the foot of the mountain, by means of the zenith telescope. For, whether there exist great caverns under the visible surface or not, the pendulum will give us a value for the mean density of the whole mass. Then, knowing the volume and density, it is a simple arithmetical process to find its influence on the plumb-line suspended at a given distance from it. The liberality of the Hawaiian government can only be commended when we find it devoting time and money to the solution of scientific questions in which the rest of the world is quite as much interested. The scope of the work was as follows: Fourteen latitude stations were to be established on the four principal islands of the group, viz: Kauai, Oahu, Maui and Hawaii. These were distributed in such a manner as to bring out the deflections of the plumb-line; being in general north and south of the high mountains. At each one of these stations it was proposed to observe thirty-three pairs of stars on three successive nights; which would bring the probable error of the resulting latitude below one-tenth of a second of arc. Practically, however, this plan could not be rigidly adhered to. At all the windward stations the weather was very unfavorable for astronomical observations. At Kohala the wind was always too strong to allow us to pitch a tent in which to live. An old sugar storehouse, long since abandoned, served instead. At Hilo, where the rain-fall is sixteen feet in one |