tions of condition, from which there resulted for the Rotation Time 9h 55m 34.2 ± 0.7, expressed in mean time. This value needs yet a small correction dependent on the latitude of the spot and also one dependent on the phase of the planet; but since the observations used are almost equally distributed on each side of opposition, this will be but small. The observations used, as well as the other observations made between these dates are published in the Monthly Notices for May, 1880. When the observations are completed we hope to make a complete discussion of them. The value of the Rotation Time here given has been arrived at by assuming that the spot had no proper motion, and is offered, of course, not with any idea of furnishing an absolute value of the Rotation Time, but for comparison with other values. The apparent motion of the spot with respect to the white spots and its fixity with respect to the dark gray masses have been spoken of before. But the question of the real motion of the spot is yet to be settled by observations. The discrepancy between the rotation time here given and the commonly accepted value has been pointed out in many of the astronomical journals. In Table II (p. 180) are collected all the determinations of the Rotation Time which I have been able to find, together with dates, observer, length of duration of spot, color, approximate Jovicentric latitude and other notes concerning the character of the spot observed. A glance at this table shows that the clear, bright spots situated within the equatorial belts and usually of short duration give a short Rotation Period from 9h 50m to 9h 53m, while the dark isolated spots farther away from the equatorial region give a period varying from about 9h 55m to 9h 56m; so that the white spots have an apparent motion with respect to the colored in the direction of the rotation. That the law of these motions may be determined by careful continued observation seems probable. In this connection there is one curious mistake which occurs in many of the semi-popular works on Astronomy which may be noted. The Rotation Period of Jupiter derived by Sir George Airy which is perhaps more often quoted than any other is given by Klein, Flammarion and others as 9h 55m 24.2. This value needs yet a correction on account of the change in the light equation during the period of observation; and the corrected definitive value 9h 55m 21.3' is given by Airy himself later on in his Memoir in Vol. IX, Mem. R. A. S. In some way the uncorrected value has been All observers of Jupiter of late years are greatly indebted to FRICTION OF LUBRICATING OILS. By C. J. H. WOODBURY, of Boston, Mass. THE resistance existing between bodies of fixed matter, moving with different velocities or directions, presents itself in the form of a passive force which results in the diminution or the destruction of apparent motion. Modern science has demonstrated that this destruction is only apparent, being merely the conversion of the force of the moving body into the oscillation of the resisting obstacle or into that molecular vibration which is recognized as heat. Direct friction refers to the case where the two bodies are in actual contact, and mediate friction where a film of lubricant is interposed between the surfaces; and it is this which applies to nearly every motion in mechanics where bodies slide upon each other. The coefficient of friction is the relation which the pressure upon moving surfaces bears to resistance. I have devoted some time to the examination of this subject, in the interests of the Mill Mutual Insurance Companies of New England. In this report of my work upon the measurement of friction of lubricating oils, I shall restrict myself to a description of the apparatus designed especially for the purpose, the method of its use, and the results obtained with a number of oils in our market which are used for lubricating spindles. Previous investigation of nine different oil testing machines used showed that none of them could yield consistent duplicate results in furnishing the coefficient of friction. operation of these machines, by their failure to obtain correct data, adduced certain negative evidence which established positive conditions as indispensable in the construction of a machine capable of measuring the friction of oils. The following circumstances must be known or preserved constant,-temperature, velocity, pressure, area of the frictional surfaces, thickness of the film of oil between the surfaces, and the mechanical effect of the friction. In addition to the foregoing conditions, the radiation of the heat generated by friction must be reduced to a minimum, and the arrangement of the frictional surfaces must be of such nature that no oil can escape until subjected to attrition. To measure the frictional resistance at the instant of a given tenperature, and at a time when both temperature and friction are varying, requires a dynamometer which is instantaneous and automatic in its action. The The apparatus consists of an iron frame supporting an upright shaft surmounted by an annular disk made of hardened tool steel. Upon the steel disk rests one of hard bronze (composed of the following alloy,-copper 32 parts, lead 2 parts, tin 2 parts, zinc 1 part) in the form of a cylindrical box. Water is fed in at one side and a diaphragm extending nearly across the interior produces a uniform circulation before discharge. Although this use of water is original with the writer in the method of its application, the first use of water to control the temperature of the bearing surfaces of oil testing machines is due to Monsieur G. Adolphus Hirn and described by him in a paper on the subject of friction, read before the Société Industrielle de Mulhouse, June 28, 1854. M. Hirn, however, confined his attention chiefly to the determination of the mechanical equivalent of heat, as measured by the amount of heat imparted to the circulating water, expressed in the work of friction. His investigations of lubrication with this apparatus were confined to the friction of lard and olive oils, at the light pressure of about one and four-tenths pounds to the square inch. Mr. Chas. N. Waite of Manchester, N. H., has independently, and I believe originally, made use of water in a friction machine, and has performed good work in the limit of his experi ments. A protection of wool batting and flannel, to guard the disks against loss of heat by radiation, diminishes the escape of heat to about two degrees per hour, which loss is not appreciable when observations are taken within a few seconds' interval. A thin copper tube closed at the lower end, reaching through the cover, extends to the bottom of the disk; the bulb of a thermometer is inserted in this tube and measures the temperature of the disks, an oil tube runs to the centre of the disk, and a glass tube at the upper end indicates the supply and its rate of consumption and also serves to maintain a uniform head of oil fed to the bearing surfaces. The rubbing surfaces of both disks were made to coincide with the standard surface plates, in the physical laboratory of the Institute of Technology, and their contact with each other is considered perfect. After this surface was finished, the bronze disk was treated with bichloride of platinum which deposited a thin film of platinum upon the surface. Upon the application of the disks to each other the steel disk rubbed off the platinum from all parts of the surface, showing the perfection of contact. This nicety of construction |