In order to determine these properties in various oils the author used a number of watch glasses, their convex side being glued to a board. Two drops of oil were placed in each watch glass and spread over its concave surface, and the board placed in a covered box in which suitable air holes had been made, and allowed to remain in a temperature varying from 21° to 37.5° C. (= 70° to 110° F.) for 100 days, and at the end of that time the results shown in table VI were noted. TABLE VI. GUMMING AND DRYING OF OILS. Temp. 21° C. to 37.5° C. = 70° F. to 110° F. Time 100 Days. --------------------------------+----------------------------- SYMBOLS ACCORDING TO TABLE III. | CONDITION. --------------------------------+----------------------------- E. K. w. | Slightly dried. W. F. N. w. | Very slightly dried. W. C. w. | Slightly gummed. B. & K. w. | No change. C. L. Co. w. | Slightly dried, and spread. C. L. Co. No. 1. | No change. Glyc. | No change. Sp. | Slightly gummed. Ol. | No change. --------------------------------+----------------------------- ~80. The Viscosity Of Oils~ denotes an approximate measurement of their relative lubricating power. Professor Thurston states[22] that "large consumers of oil sometimes purchase on the basis of this kind of test solely. It is regarded as satisfactory and reliable as any single physical or chemical test known, and is second only to the best testing machine methods. The less the viscosity, consistently with the use of the oil under the maximum pressure to be anticipated, the less is, usually, the friction. The best lubricant, as a rule, is that having the least viscosity combined with the greatest adhesiveness. Vegetable oils are more viscous than animal, and animal more so than mineral oils. _The fluidity of an oil is thus, to a large extent, a measure of its value._" The relation between the viscosity and the friction reducing power of oils has been determined by Mr. N. C. Waite[23] and others to be very close. An oil having little viscosity is suitable for the escapement and lighter parts of the train, but is not a good lubricant for the bearings of the center pinion and barrel arbor and the mainspring, which require a more viscous lubricant; while a still greater viscosity renders it more serviceable on the stem winding mechanism (59) and in the pendant (60). Again, an oil that possesses sufficient "body," or combined capillarity (32) and viscosity, to resist the tendency to be "squeezed" from between the bearing surfaces in the heavier parts of the mechanism will produce a _great excess of fluid friction_ in the lighter parts of the train and in the escapement.