The Instrument is this. I prepare a pretty capaceous Bolt-head AB, with a small stem about two foot and a half long DC; upon the end of this D I put on a small bended Glass, or brazen syphon DEF (open at D, E and F, but to be closed with cement at F and E, as occasion serves) whose stem F should be about six or eight inches long, but the bore of it not above half an inch diameter, and very even; these I fix very strongly together by the help of very hard Cement, and then fit the whole Glass ABCDEF into a long Board, or Frame, in such manner, that almost half the head AB may lye buried in a concave Hemisphere cut into the Board ...