~83. The Effect Of Cold On Oils~ is very observable in some varieties, converting them into greases, or even into hard, waxy solids. For out-of-door work unguents must be selected that will "feed" at any temperature to which they are exposed in the working of the bearings to which they are applied. The author has subjected various oils to a low degree of temperature, using a sufficient number of thin glass test tubes of 3 cubic centimeters capacity,[24] into each of which 2 cubic centimeters of the oils to be tested were poured. The test tubes were then tightly corked and properly secured to a thin board, and placed in a temperature of -15° C. (= 5° F.) the condition of the oils being noted at various intervals, the result of which is shown in table X. ~84. The Variations of Viscosity of Oils in Varying Temperatures~ always create fluctuations of their friction reducing power; while the variations of fluid friction which result are also of great importance in horology. When it is known that the viscosity and lubricating power of an oil are usually (80) very closely related, it is seen that change of temperature has an exceedingly important effect upon oils, even for general lubricating purposes; but particularly so when they are applied to small and delicate mechanisms. An oil of the proper viscosity at ordinary temperatures may be very unsuitable in an extreme of heat, or cold, to which timepieces are frequently subjected--on account of being too limpid in high temperatures to properly separate the rubbing surfaces; while in low temperatures it may become so viscous as to seriously impede the motion of the escapement and the lighter parts of the train. TABLE X. RELATIVE EFFECT OF COLD ON OILS. Temp. -15° C. (= 5° F.) Time of Exposure = 6 hours