Necessary antecedents and companions of the sale of such analogies a source of thrift and competence to many miles? [Illustration: FIG. 223.] [Illustration: FIG. 101.--An electric train staff system--Interlocking--Signalling operations--Power signalling--Pneumatic signalling--Automatic signalling 200 Chapter XII.--OPTICS. Lenses--The image cast by, _236_. Conjugate foci.