This power may be supplied by a steam engine; but it must not be forgotten that the prime cost and running expenses of such an engine are considerable, and form an important item in view of the low value of most earth colours. Consequently, it is highly important to be able to generate motive power as cheaply as possible. Now, the cheapest and most uniform source of power is water; and therefore, wherever the conditions allow of the erection of the colour works near a stream or river, which can supply the power to run the various machinery, the most favourable circumstances will have been secured, the power being obtained at minimum cost, whilst the upkeep of the motor cannot be very great. If there is sufficient head for the water to be run through a trough over the top of the levigation tanks, the conditions will be ideally favourable. Wind power costs nothing, once the motor has been installed; but unfortunately, one is dependent on the weather, and sometimes there is not enough wind, for days together, to drive the sails at all, and therefore all the operations have to be stopped, including levigation, the water for which has to be raised by a windmill pump. In districts where the winters are severe, water power may also fail and work have to be stopped; and consequently, even when water power is the prime source of energy, a steam engine must be installed as a stand-by, being, of course, only used when the main source of power gives out or proves insufficient. The machines employed for preparing the raw materials in the manufacture of earth colours may be divided into the following groups:-- Machines operating entirely by pressure: crushers; machines acting by impact: stamps; those acting by impact and pressure: vertical mills (edge-runners), ball mills, centrifugal mills; and, finally, machines with a frictional action: grinding mills. Then there are the levigating machines, which do not reduce the material but separate the coarser particles from the finer. The construction of the foregoing machines is a matter for the machinery manufacturer rather than the maker of earth colours; but as the business of the latter is dependent on them, a short description is considered necessary. The selection depends, on the one hand, on the nature of the materials to be treated, and, on the other, on the size of the works, since a manufacturer who has to deal with large quantities of a given raw material will require different machines from those used on a small scale. The sole purpose of the following description is to indicate to the colour maker the way in which the reduction of the raw material can be accomplished. CRUSHING MACHINERY