Cleansing old paintings
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C.
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Article
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1884
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Elsevier Science
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English
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## 249 professor, that when I manil)ulated I had some means of increasing its friction, whereby I couht control the amont of fi'iction. I asked him how ? For instance, he said~ it is quite possible for the ball to be nmde with two holes in it, one passing through the axis of the sphere in a right