F = 1·44 + 0·252 R.
APPENDIX II.
DETAILS OF THE WILLIS APPARATUS USED IN
ILLUSTRATING THE FOREGOING LECTURES.
The ultimate parts of the various contrivances figured in this volume are mainly those invented by the late Professor Willis of Cambridge. They are minutely described and illustrated in a work written by him for the purpose under the title System of Apparatus for the use of Lecturers and Experimenters in Mechanical Philosophy, London, Weale & Co., 1851. This work has long been out of print. It may therefore be convenient if I give here a brief account of those parts of this admirable apparatus that I have found especially useful The illustrations have been copied from the plates in Professor Willis’ book.[2]
The Willis system provides the means for putting versatile framework together with or without revolving gear for the purpose of mechanical illustration. Many parts which enter into the construction of the machine used at the lecture to-day will reappear to-morrow as essential parts of some totally different contrivance. The parts are sufficiently substantial to work thoroughly well. The scantlings and dimensions generally have been so chosen as to produce models readily visible to a large class.
It will of course be understood that every model contains some one or more special parts such as the punch and die in [Fig. 73], or the spring balance in [Fig. 17], or the pulley-block in [Fig. 33]. But for the due exhibition of the operation of the machine a further quantity of ordinary framework and of moving mechanism is usually necessary. This material, which may be regarded as of a general type, it is the function of the Willis system to provide.
The Bolts.—The system mainly owes its versatility and its steadiness to the use of the iron screw bolt for all attachments. The bolts used are ⅜ diameter; the shape of the head is hemispherical and the shank must be square for a short distance from the head so that the bolt cannot turn round when passed through the slits of the brackets or rectangles. When the head of the bolt bears on a slit in one of the wooden pieces a circular iron washer 2" in diameter, or a square washer 2" on each side, is necessary to protect the wood from crushing. There is to be a square hole, in the washer to receive the square shank of the bolt and the thickness of the washers should be ⅛". The nut is square or hexagonal, and should always have a washer underneath when screwed home with a spanner or screw-wrench. The most useful lengths are 2", 4", 6". The proper kind are known commercially as coach-bolts, and they should be chosen with easy screws, for facility in erecting or modifying apparatus. At least two dozen of the intermediate size and a dozen of each of the others are required. For elaborate contrivances many more will be necessary.