is about £4 per indicated horse-power, and the efficiency under the best conditions, namely, with full load, is, according to Professor Ayrton, as high as 96 per cent., and when it is doing one quarter of the full work 89 per cent. APPLICATION OF TRANSFORMERS. The installation at the Grosvenor Gallery, London, may be taken to illustrate Class I. or the practical working of distribution by means of transformers. Fig. 11 represents the arrangement of primary and secondary circuits. An alternating current is sent through the main L L¹, which is a closed circuit, and a small portion is drawn off wherever there is a secondary generator or transformer T; these instruments are placed in parallel between the conductors in the same manner as a glow lamp; neither main can be called positive or negative, as the current flows backwards and forwards many times in a second. The house wires M M are joined to the secondary circuits, and are quite distinct from the main, which they do not even touch, although sufficiently near to receive an induced current alternating the same as the primary, but of a much lower electro-motive force. [Illustration: FIG. 11. D, alternating current dynamo; E, continuous current dynamo for exciting; L L¹, main primary conductors; M M, secondary conductors; T T, transformers; S¹ S² S³ S⁴ S⁵ S⁶, lamps in parallel.] The Goulard transformers were used at first, but have been superseded by others designed by Mr. Ferranti; they are of the No. 2 kind, or shell type, and have a core of hoop-iron, on which the two coils are wound; the hoop-iron is then bent over, and the ends joined so as to enclose the coils. The machinery is fixed in a basement excavated under the Grosvenor Gallery; the foundations are of massive concrete, in which stone supports for the engines and dynamos are embedded; the concrete does not touch the walls of the building, but a space of about 1 foot is left, which is filled in with clay; and by this simple plan all vibration of the machinery is isolated from the building. The power is obtained from two horizontal high-pressure engines, each of 600 indicated horse-power, in addition to the original two horizontal high-pressure non-condensing engines, each of 35 nominal horse-power, running at a speed of 55 revolutions per minute, which is maintained constant by means of a governor directly attached to the expansion slide-valve. The four engines drive on to a countershaft, which is cut up into lengths; each section is coupled to a dynamo and exciter by means of a conical friction-disk clutch; this permits of either length being started or stopped without interfering with the other. The speed of each engine is checked by means of a liquid speed-indicator, designed by the author. Two Ferranti alternating current dynamos, each capable of maintaining ten thousand lamps, are driven direct, one dynamo by each length of shafting: they are excited by two