536 ON THE LUBRICATING AND OTHER [429
slab was used in the horizontal position and the movable piece was pulled by a thread which started horizontally, and passing over a pulley carried a small pan into which weights could be placed. The pan itself weighed 1 oz. (28 grams). Another change was the substitution for the bottle of a small carriage standing on glass legs terminating in three feet of hemispherical form and 5 mm. in diameter. The whole weight of the carriage, as loaded, was 7f oz. The object of the substitution was to eliminate any effects which might arise from the comparatively large area of approximate contact presented by the rim of the bottle, although in that case also the actual contacts would doubtless be only three in number and of very small area.
With J oz. in pan and surfaces treated with the hand, the carriage would move within a second or two after being placed in position, but after four or five seconds' contact would stick. After a few minutes' contact it may require 1£ oz. in pan to start it. When the slab is breathed upon it requires, even at first, 3^ oz. in the pan to start the motion. As soon as the breath has evaporated, £ oz. in pan again suffices. When the weight of the pan is included, the forces are seen to be as 1:3. When the feet stand in a pool of water the stickiness is nearly the same as with the breath, and the substitution of soapy for cleaw water makes little difference.
In another day's experiment paraffin (lamp) oil was used. After handling, there was free motion with 1 oz. in pan. When the feet stood in the oil, from 2f to 3 oz. were needed in the pan. Most of the oil was next removed by rubbing with blotting-paper until the slab looked clean. At this stage f oz. in pan sufficed to start the motion. On again wetting with oil 2 oz. sufficed instead of the 2| oz. required before. After another cleaning with blotting-paper \ oz. in pan sufficed. From these results it appears that the friction is greater with a large dose than with a minute quantity of the same oil, and this is what is hard to explain. When olive oil was substituted for the paraffin oil, the results were less strongly marked.
Similar experiments with a carriage standing on brass feeb of about the same size and shape as the glass ones gave different results. It should, however, be noticed that the brass feet, though fairly polished, could not have been so smooth as the fine surfaces of the glass. The present carriage weighed (with its load) 6£ oz., and on the well-handled glass slide moved with £ oz. in pan. When the slide was breathed upon, the motion was as free as, perhaps more free than, before. And when the feet stood in a pool of water, there was equal freedom.. A repetition gave confirmatory results. On another day paraffin oil was tried. At the beginning £ oz. in pan sufficed on the handled slab. With a pool of oil the carriage still moved with ^ oz. in pan, but perhaps not quite so certainly. As the oil was removed with blotting-paper the motion became freer, and when the oil-film had visibly disappeared the £ oz. in pan could about be dispensed with. Doubtless a trace of oil remained.t was subsequent to Sonstadt's.