measuring the track of the oil. The length of time consumed in its descent is the measure of its viscosity. This form of apparatus also allows of observing any tendency of the oil to gumming," which is here permitted by the oxidizing properties of the air.

Besides the determination of the velocity of the flow of an oil, there have recently been constructed viscoscineters based upon different principles, of which J. C. Stahl's apparatus may be taken as a representative. It consists of a glass tube which can be closed on both ends by glass-stoppers. A short distance below these stoppers the tube is provided with marks so that when the tube closed at one end is filled with the oil to be tested and the other glass stopper is inserted, there remains an air-bubble of a fixed size between the stopper and the mark. By holding the tube in a perpendicular position and then turning it 180° so that the glass stopper previously on top comes below, the viscosity can be measured by the time required by the air-bubble to ascend in the oil. The more thickly-fluid the oil is, the more slowly the bubble ascends.

Manufacturers who are close observers have a variety of simpler tests, such as timing a ball falling through an oil column of known height; rubbing a portion of the oil between the thumb and forefinger; letting drops of the oil fall upon the surface of the same oil in bulk; exposing the oil for a few hours, or a day, upon the surface of a piece of glass. These tests (except perhaps the first) are not accurate, and oils are not bought or sold upon their indications.

Tests as to frictional resistance.

Besides testing lubricating oils as to their viscosity, it is also of importance to determine, by direct lubricating experiments, their frictional resistance. The power of oils to decrease friction depends, under all conditions, on their degree of fluiditytheir viscosity. The more thinly-fluid an oil is, the slighter its frictional resistance. Hence, a good oil should be as thinlyfluid as possible, but at the same time possess as much adhe

sion as possible to form a sufficient layer between the rubbing parts.

The extent to which friction is decreased by a lubricant may be established either directly by determining the co-efficient of friction, or indirectly by the heating which a bearing lubricated with the oil to be tested shows in a determined number of revolutions.

Many apparatuses serving for the determination of the coefficient of friction admit also of forming a judgment regarding the chemical behavior (thickening, gumming) of a lubricant under the influence of strong friction; of the consumption of material, as well as of the lubricating power at various degrees of temperature. In the absence of an absolute measurement of the lubricating power all results are only relative, i. e., they only show the extent to which the material tested deviates from a lubricant of noted good quality, which serves as a standard. The experiments may be executed in accordance with two different principles. The rubbing surfaces (plates, bearings and axles) may be provided with a quantity of oil just sufficient for lubrication, and, besides the co-efficient of friction or increase in the temperature of the rubbing parts, the time may be determined, which, with a minimum quantity of oil used in the experiment, is required for the total consumption of the oil; or, an excess of oil may be employed and the co-efficient of friction or increase in temperature which shows itself within a certain space of time may be determined.

Up to the present time no apparatus for determining the lubricating power of oils has been constructed, which answers all requirements. The defects inherent in almost all such apparatus are due to the technical difficulty of constructing rubbing surfaces unalterable under great pressure and strong heating, and which remain constant, especially when thick and thin oils are to be successively treated. There is the further difficulty of uniformly distributing the oil to be tested between the rubbing surfaces without a portion of it being forced out, and the losses of heat in consequence of the conduction of the metallic parts, and partially of radiation.

The best results are obtained by always using the apparatus on hand under as nearly equal conditions as possible, that is, by testing oils not differing too much in consistency at a not too varying pressure of the rubbing surfaces, and at a nearly equal revolving velocity of the axles, etc., as well as at nearly the same temperature of the air. Of the separate apparatuses some, in consequence of their construction, are more suitable for light, and others for heavy, oils.

It may here be remarked that in regard to the results obtained by testing the same oils with different apparatuses no standard can be set up, the construction of the apparatuses varying too much, and the conditions under which the experiments are made being never almost alike, so that a standard has to be established for each apparatus, i. e., the results obtained with the oils tested have to be referred to those obtained with an oil selected as a standard.

The most important apparatuses for testing the lubricating power of the oils will here be described:

To the oldest completely arranged machines of this kind belong the oil-testing apparatus of MacNaught of Glasgow, and that of M. Hogson. Both machines are based upon the same principle, and differ only in size and in the execution of meas uring the magnitude of friction. The oils are tested as follows:

The oil to be tested is interposed between two horizontal disks, the lower one of which sits fast upon a vertical shaft and revolves with it, while the upper one is loose and revolves only by the friction caused between the two disks. The motion of the upper disk is checked by a special contrivance, and the resistance which it opposes to this checking is equal to the friction between the two disks. With Mac Naught's machine the friction is measured by allowing the resistance to act upon a T lever. A movable weight slides on the arm, on which is a scale to note its position. A counterweight is attached to the opposite end of the lever so as to afford the means of a more delicate adjustment.

With Hogson's machine the resistance of the disk is kept in equilibrium by a spring balance, and the friction is measured

by the tension of the latter. This machine is so arranged

that the pressure to which the lubricant between the two disks

is exposed, can be increased to within certain limits, and that the velocity of revolution can be changed at will.

Woodbury's apparatus for testing lubricants. This machine is shown in perspective in Fig. 165, and in elevations in Figs. 166 and 167. The lower disk is secured upon the top of an upright shaft, its top being an annulus, ground to a true plane surface. Upon this rests the upper disk, which is in the form of a hollow ring, based upon a flat plate, and is made of a very hard composition cast in one piece. The bottom of this disk is scraped to a true plane surface, so that the contact between these two disks is uniform.

A partition divides the interior of the hollow ring, forming the upper disk, so that water can be introduced through the connecting tubes to control the temperature of the disks or to retain the heat of friction. The sides and top of the upper disk are surrounded by a case of hard rubber, and the space is filled in with eider down.

Ice water is generally used to reduce the temperature of the disks nearly to the freezing point of water, and the friction is noted at each degree of the rise in temperature due to the heat of friction.

A tube of thin copper, closed at the bottom, reaches through the bottom of the disk, and a thermometer with its bulb placed within this tube, indicates the temperature of the frictional surface. A tube leading through the upper disk conducts the lubricant under trial to a recess in the middle of the lower disk. The upper end of this tube being of glass, indicates the supply and rate of feeding of the oil. As the friction of a journal depends quite largely upon the method of lubrication, uniformity in the manner of supply is of the utmost importance.

Over the upper disk a yoke with four arms rests upon four columns, which extend through the upper disk to the middle of the frictional surfaces, these columns being cast as a portion of the disk. In the centre of this yoke is a hole with hemispherical

1 Trans. of Am. Soc. Mech. Engineers, vol. vi., p. 136, Nov., 1884.