17. It sometimes happens that cylindric or globe machines do not work well, owing to the air within them being too much rarefied by the heat of the cement, when the caps are fixed on. To remedy this, a small hole may be bored through one of the caps, so as to admit air into the cylinder or globe. 18. If the electric by any means become scratched, the working of the machine will be greatly impeded, if not altogether prevented. This is accounted for upon the principle that smooth and rough glass electrify differently when excited by the same rubber, and the two different states destroy one another. This may be remedied by filling up the scratches with a little tallow. EPITOME OF ELECTRICITY. DIVISION II. CHAP. I. _Entertaining Experiments, made by electrical Attraction and Repulsion._ Electric attraction and repulsion, observed in excited amber, was, as we have already had occasion to remark, the first phenomenon which was noticed in the science of which we treat. We have also hinted that, to the present time, no explanation of this attraction and repulsion which is entirely satisfactory, has been given. Facts, however, are known in abundance; and certain principles, relative to this part of our subject, are clearly ascertained. To exhibit and illustrate these has been our object in selecting the following experiments; some of which may be considered as intended chiefly for amusement, but all of which, if thoroughly comprehended, will serve to fix the principles of the science more deeply in the mind of a learner. EXPERIMENTS. _The self-moving Wheel._