The different kinds of motive power used to actuate cranes--manual, steam, hydraulic, electric--give a further classification. Hand cranes are extremely useful where the load is not excessive, and the quantities to be dealt with are not great; also where speed is not important, and first cost is an essential consideration. The net effective work of lifting that can be performed by a man turning a handle may be taken, for intermittent work, as being on an average about 5000 foot-lb per minute; this is equivalent to 1 ton lifted about 2¼ ft. per minute, so that four men can by a crane raise 1 ton 9 ft. in a minute or 9 tons 1 ft. per minute. It is at once evident that hand power is only suitable for cranes of moderate power, or in cases where heavy loads have to be lifted only very occasionally. This point is dwelt upon, because the speed limitations of the hand-crane are often overlooked by engineers. Steam is an extremely useful motive power for all cranes that are not worked off a central power station. The steam crane has the immense advantage of being completely self-contained. It can be moved (by its own locomotive power, if desired) long distances without requiring any complicated means of conveying power to it; and it is rapid in work, fairly economical, and can be adapted to the most varying circumstances. Where, however, there are a number of cranes all belonging to the same installation, and these are placed so as to be conveniently worked from a central power station, and where the work is rapid, heavy and continuous, as is the case at large ports, docks and railway or other warehouses, experience has shown that it is best to produce the power in a generating station and distribute it to the cranes. Down to the closing decades of the 19th century hydraulic power was practically the only system available for working cranes from a power station. The hydraulic crane is rapid in action, very smooth and silent in working, easy to handle, and not excessive in cost or upkeep,--advantages which have secured its adoption in every part of the world. Electricity as a motive power for cranes is of more recent introduction. The electric transmission of energy can be performed with an efficiency not reached by any other method, and the electric motor readily adapts itself to cranes. When they are worked from a power station the great advantage is gained that the same plant which drives them can be used for many other purposes, such as working machine tools and supplying current for lighting. For dock-side jib cranes the use of electric power is making rapid strides. For overhead travellers in workshops, and for most of the cranes which fall into our second class, electricity as a motive power has already displaced nearly every other method. Cranes driven by shafting, or by mechanical power, have been largely superseded by electric cranes, principally on account of the much greater economy of transmission. For many years the best workshop travellers were those driven by quick running ropes; these performed admirable service, but they have given place to the more modern electric traveller.