moderate weather once in every twelve or fifteen seconds. It is true that the moon in its attraction of the sea-water produces a vastly greater sum total of effect than the wind does in raising the surface-waves, but reckoning only that part of the ocean energy which might conceivably be made available for service it is safe to calculate that the waves offer between two and three thousand times as much opportunity for the capture of natural power and its application to useful work as the tides could ever present. In no other form is the energy of the wind brought forward in so small a compass or in so concrete a form. A steam-ship of 10,000 tons gross weight which rises and falls ten times per minute through an average height of 3·3 feet is thereby subjected to an influence equal to 22,400 horse-power. In this estimate the unit of the horse-power which has been adopted is Watt's arbitrary standard of "33,000 foot pounds per minute". The work done in raising the vessel referred to is equal to ten horse-power multiplied by the number of pounds in a ton, or, in other words, 22,400 horse-power, as stated.

Wind-power, again, has been to a large extent neglected since the advent of the steam-engine. The mightiest work carried out in any European country in the early part of the present century was that which the Dutch people most efficiently performed in the draining of their reclaimed land by means of scores of windmills erected along their seaboard. Even to the present day there are no examples of the direct employment of the power of the wind which can be placed in comparison with those still to be found on the coasts of Holland. But, unfortunately for the last generation of windmill builders, the intermittent character of the power to which they had to trust completely condemned it when placed in competition with the handy and always convenient steam-engine. The wind bloweth "where it listeth," but only at such times and seasons as it listeth, and its vagaries do not suit an employer whose wages list is mounting up whether he has his men fully occupied or not. The storage of power was the great thing needful to enable the windmill to hold its own. The electrical storage battery, compressed air, and other agencies which will be referred to later on, have now supplied this want of the windmill builder, but in the meantime his trade has been to a large extent destroyed. For its revival there is no doubt that, as Lord Kelvin remarked in the address already quoted, "the little thing wanted to let the thing be done is cheap windmills."

This, however, leads to another part of the problem. The costliness of the best modern patterns of windmill as now so extensively used, particularly in America, is mainly due to the elaborate, and, on the whole, successful attempts at minimising the objection of the intermittent nature of the source of power. To put the matter in another way, it may be said that lightness, and sensitiveness to the slightest breeze, have had to be conjoined with an eminent degree of safety in the severest gale, so that the most complicated self-regulating mechanisms have been rendered absolutely imperative. Once the principle of storage is applied, the whole of the conditions in this respect are revolutionised. There is no need to attempt the construction of wind-motors that shall run lightly in a soft zephyr of only five or six miles an hour, and stability is the main desideratum to be looked to.

The fixed windmill, which requires no swivel mechanism and no vane to keep it up to the wind, is the cheapest and may be made the most substantial of all the forms of wind-motor. In its rudimentary shape this very elementary windmill resembles a four-bladed screw steam-ship propeller. The wheel may be constructed by simply erecting a high windlass with arms bolted to the barrel at each end, making the shape of a rectangular cross. But those at one end are fixed in such positions that when viewed from the side they bisect the angles made by those at the other side. Sails of canvas or galvanised iron are then fastened to the arms, the position of which is such that the necessary obliquity to the line of the barrel is secured at once.

Looking at this elementary and at one time very popular form of windmill, and asking ourselves what adaptation its general principle is susceptible of in order that it may be usefully employed in conjunction with a storage battery, we find, at the outset, that, inasmuch as the electric generator requires a high speed, there is every inducement to greatly lengthen the barrel and at the same time to make the arms of the sails shorter, because short sails give in the windmill the high rate of speed required.

We are confronted, in fact, with the same kind of problem which met the constructors of turbine steam-engines designed for electric lighting. The object was to get an initial speed which would be so great as to admit of the coupling of the dynamo to the revolving shaft of the turbine steam-motor, without the employment of too much reducing gear. In the case of the wind-motor the eighteenth century miller was compelled to make the arms of his mill of gigantic length, so that, while the centre of the wind pressure on each arm was travelling at somewhere near to the rate of the wind, the axis would not be running too fast and the mill stones would never be grinding so rapidly as to "set the tems—or the lighter parts of the corn—on fire."

The dynamo for the generation of the electric current demands exactly the opposite class of conditions. We may therefore surmise that the windmill of the future, as constructed for the purposes of storing power, will have a long barrel upon which will be set numerous very short blades or sails. Reducing this again to its most convenient form, it is plain that a spiral of sheet-metal wound round the barrel will offer the most convenient type of structure for stability and cheapness combined. At the end of this long barrel will be fixed the dynamo, the armature of which is virtually a part of the barrel itself, while the magnets are placed in convenient positions on the supporting uprights. From the generating dynamo the current is conveyed directly to the storage batteries, and these alone work the electric motor, which, if desired, keeps continually in motion, pumping, grinding, or driving any suitable class of machinery.

It is rather surprising to find how relatively small is the advantage possessed by the vane-windmill over the fixed type in the matter of continuity of working. During about two years the Author conducted a series of experiments with the object of determining this point, the fixed windmill being applied to work which rendered it a matter of indifference in which way the wheel ran. With the prevailing winds from the west it ran in one direction, and with those of next degree of frequency, namely from the east, it turned in the reverse direction. The mill, however, was effective although the breeze might veer several points from either of the locations mentioned. It was found that there were rather less than one-fourth of the points of the compass, the winds from which would bring the wheel to a standstill or cause it to swing ineffectively, but as these were the directions in which the wind least frequently blew it might safely be reckoned that not one-eighth of the possible working hours of a swivel-windmill were really lost in the fixed machine.

With the type adapted to the working of a dynamo as already described, it will, in most cases, be convenient to construct two spirals on uprights set in three holes in the ground, forming lines at right angles to each other, but both engaging, by suitable gearing, with the electric