straight line and parallel to the surface of the paper. Now, while the strip of paper, as soon as the pivot is made to revolve, moves in the direction of its length, the pencil changes its position, according to the heat produced in the arbor, at a right angle to the direction of the motion of the paper, and its markings, every fifteenth second, form points of a curve which gives a general picture (Fig. 181) of the development of heat in the layer of lubricant and in the bearing and arbor.

The curve marked by the pencil soon forms a straight line, running parallel to the direction of the motion of the paper, because the temperature of the arbor remains constant very soon after revolution begins, that is to say, when the effect of the

FIG. 180.

FIG. 181.

b

a

a, temperature of air; b, permanency.

cooling off of the surrounding air is just as large as that of the only slight development of heat in the still fresh layer of lubricant. This condition may suitably be called "permanency."

The distance of the degrees designating this condition from the degrees corresponding to the temperature of the air is differently given by this apparatus for the various kinds of lubricants, but always exactly alike for the same kind. The curve thus formed has the shape as shown in Fig. 181.

Now, by allowing the arbor to revolve sufficiently long, the material adhering to it will be constantly reconveyed to the place between arbor and bearing, and repeatedly exposed to

the work originating there. As the quantity of lubricant remains, however, the same, the effects of the work sum up so that the temperature of the arbor no longer remains constant, but rises proportionally to the internal resistances of work of the lubricant itself. The curve in its further course then takes an upward direction as indicated by dots.

The more quickly the lubricant changes its normal condition, the more resisting heat it develops; and the greater its consumption, the more rapidly the curve ascends.

From the previously explained mechanical combination of the machine it will be seen that the abscissa of the several points of this curve represent the revolution of the arbor, and consequently also the road traversed from the centre of gravity of the layer of lubricant between the bearing and arbor, while the ordinates of the changes in the condition of the layer express its heating and the magnitude of its injurious internal resistances according to quantities of heat in corresponding lengths of the ordinates.

The curve, therefore, represents a picture of the relations between the road and resistance within one and the same period, and the product of both consequently gives the magnitude of that work of resistance which the lubricant has developed in itself by the demand between arbor and bearing.

This product is directly represented by the area terminated by the curve itself and the abscissa-the line of the temperature of the air-passing through its point of beginning.

With different lubricants the internal work of resistance is, therefore, under otherwise equal circumstances, proportional to these areas, and their mechanical durability inversely proportional to them.

In this manner it is possible to express the mechanical effect of a lubricant as compared with another by a fixed value of figures which makes it also possible to answer the question. under § 2, page 498.

It may here be remarked that experiments with the described testing machines are of value only within certain limits, and

that a comparison of the results obtained with the different apparatuses is impossible. Even the results of two machines of the same construction cannot, according to Engler, be compared with each other, since the slightest variations in the constitution (shape, hardness, etc.,) of the rubbing surfaces give rise to considerable differences in testing one and the same oil. Even the results with the use of one and the same apparatus are not uniform and cannot be relied on, if the directions for making the experiments are not strictly observed.

It is impossible to obtain exact, useful results if the test-surfaces are not most accurately worked, for instance, if the axles are not almost absolutely round, or are not placed in a perfectly horizontal position and firmly secured; further, if the rubbing metal is not possessed of sufficient solidity and hardness to remain perfectly constant under the influence of friction. According to observations by Engler, no testing machine is entirely free from one or the other of the above-mentioned defects, and, in fact, a machine answering all requirements does not exist. By further taking into consideration that the absolutely necessary conditions in making experiments are freqently not observed with sufficient nicety, it is evident that the results of experiments regarding the lubricating power of different kinds of oil, which have from time to time been published, are of little or no value.

Experiments made with the various testing-machines always lead to the same results, viz., that the coefficient of friction stands in direct relation to the viscosity of the oils; thinly-fluid oils, i. e., oils of slight viscosity, have a small coefficient of friction and will not bear a great load, while thick oils show, with a light load, a large coefficient of friction, but will also stand a heavy load. The latter, however, can be advantageously used for lubricating purposes only when the axle and bearing are warm enough to render them more thinly-fluid, this being the reason why thick oils at first yield a large coefficient of friction, which constantly becomes smaller as the axle and bearing become warmer, and finally remains quite constant.

From the results of Lamansky's experiments (see p. 493) it is evident that in consequence of more uniform viscosity at varying temperatures, vegetable oils also possess more uniform and constant lubricating power than mineral oils, whose viscosity rapidly decreases with a rising temperature. It has further been shown that the friction with one and the same oil is considerably increased with an increase in the velocity of the rubbing surface, and that when lubrication is effected by means of wick the co-efficient of friction is considerably greater than when there is a constant excess of oil between the axle and bearing.

In regard to directly testing oils as to their lubricating power, Engler arrived at the following conclusions, which are based upon numerous experiments made in the laboratory by himself and others:

1. To judge of the value of a lubricant for a definite purpose, it is advisable to use a testing machine whose construction and other conditions correspond as closely as possible with the intended practical use. Thus, for instance, for railroad purposes, the testing machine of the Paris-Lyons Mediterranean Railway might be recommended.

2. Generally speaking, testing machines give the most reliable results when they are used within narrow experimental limits (pressure and temperature). The execution of successive experiments under very extreme conditions of pressure and with oils varying very much in their degree of fluidity should be avoided.

3. The direction of the revolving surfaces should always be the same. If, for instance, with Thurston's apparatus, the axle is allowed to run in a direction reverse to the ordinary, considerably stronger frictional effects will result.

4. Testing machines whose rubbing surfaces become strongly heated give unreliable results for the direct measurement of the frictional resistance, and hence it follows that machines whose axles and bearings exhibit large surfaces of contact are also to be excluded. It would be best for the contact to take

Tace of Dre ine par TI E Ly one side (above Kievit a tee the co-efcent of friction alone is 20 be determined a costic de ribong test-portions hollow and T. les Ter umgenare constant by a fuid.

3 String texting Des-is, beans, etc., constitutes a de I In one reason that that the oils are thereby tested mom 2000TL 15-comanies. It is also a source of enfor Decause will a rating temperature of the trial-room an regla CH IT IS Cised by radance and conduction.

6 Or the corky Is enige that is whose lubricating pover is demanded a high temperature, must also be tested temperature

For the compensation of avoidable construction-errors to the results of the experiments, the test-conditions (duration of the experiment number of revolutions, load, temperature of the surrounding air quantity to should, for all comparative experiments. be as nearly the same as possible. These testconditions have to be adapted as much as possible to the intended use pressure and velocity of the rubbing surfaces).

8. If the co-eficient of friction alone is to be determined, and, in general, the heating which eventually takes place, the test has to be made with an excess of the oil under trial. Only in special cases is the supply of oil to be adapted as much as possible to the intended practical use, for instance, by supplying the oils by means of wick.

9. For the determination of the durability of a lubricant comparative tests have to be made always with the same quantities of oil, it being best to use no more than is necessary for once lubricating the rubbing surfaces.

10. The rubbing surfaces of the testing machines should be worked with the utmost accuracy, polished as smoothly as pos sible, and be of such hardness that they are not changed by the experiments.

11. The viscosity of oils is so closely related to their lubri cating power that, if the determinations have been made at the respective temperatures, conclusions as to the usefulness and