270. The top revolving screen of the stone-breaker is usually constructed so as to produce two different sizes of macadam, and a division in the delivery-shoot provided for this purpose enables each class to be removed at will.

By this arrangement the smaller-sized macadam can be used for patching or for coating the hilly parts of the highways, but it is seldom the case that both sizes of macadam can be consolidated separately on the same road.

It therefore becomes necessary to store one or other class of macadam, which, on economical grounds, should be avoided as much as possible.

This will be apparent when it is considered that to store and refill road metal, an expenditure of from 3d. to 5d. per ton is incurred.

It is more usual, however, to have different sets of top revolving screens having perforations of a size suitable for the work on hand, so that the total amount of macadam broken may be removed direct to the roads under repair.

For general purposes a top screen having holes 2 inches in diameter will suit the requirements and conditions of work generally, and may save the employment of other screens having a mesh of a larger or less diameter, provided the method by which the renewal or repair of the surface of the roads is properly considered, and the necessary work is arranged for in a practical manner.

The stone-breaking machine which has been described fully in the foregoing pages is the only practical machine, so far as the author is aware, combining the simplest form of construction for the efficient operations of breaking, screening, and loading. The detailed amount of work which can be accomplished by its use, and the working expenses involved in the production of macadam, are such as will compare favourably with any other make of machine for similar purposes, while the motive power necessary to work it is considerably less than in the case of most of the other stone-breakers. The latest design of this stone-breaker is shown in fig. 106.

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271. Marsden of Leeds and Mason of Leicester make a form of screening and loading machine, which is so arranged that all the material broken is elevated overhead to the top revolving screen, where the dust, chips, and macadam are separated. In working a breaking machine with this arrangement it is obvious that the dust treated at such a height will be blown in every direction during the prevalence of wind, causing great inconvenience to the men engaged in working the machine. The increased carrying capacity required for the elevator belt will necessarily involve a greater motive power to overcome the friction caused by this additional weight of material being raised.

A great variety of the jaw type of stone-breaking machines are in use in the United States and other countries, all more or less constructed on the same principle as the original Blake. One form of stone-breaker differing widely, in the principle of construction and working, from the type of machines known in this country has, it is stated, been performing record work in America.

272. Gates' 'Stone Crusher.'-This machine is known by the name of the Gyratory Rock and Ore Crusher,' and is manufactured by the Gates Iron Works of Chicago. As the name implies, the mechanism is such that the crushing cone,' fitted on to a vertical or main shaft with a certain amount of eccentricity, impinges during its gyration against the material to be broken, and also approaches to and recedes from the sides of the enclosed cylindrical shell. The necessary power for operating the crusher is transmitted by bevel gearing and pinion, and is driven direct by a belt and pulley fixed on the counter shaft.

It works without the intervention of fly-wheels, such as occur in the other types of stone-breaking machines, and it is claimed that under working conditions there is little or no vibration.

It is of simple construction, and has few working parts. The strain on the gearing, however, when running at a high speed, must necessarily be very great, from which it is reasonable to suppose that great wear and tear and occasional breaking of these parts will take place. This machine, although specially designated a 'crusher,' would appear, from the construction and movement of the working parts, to create a sliding motion which would naturally subject the material to crushing. This, however, does not occur to any great extent in actual practice, the machine turning out a very fair sample of macadam.

A vertical section of the gyratory machine, known as the 'D type,' which is the latest form of crusher made by this firm, is shown in fig. 107 fixed on a timber foundation.

The principal parts of the crusher may be described as follows:-The shaft A is held rigidly at the top in the spider J, while it is gyrated by means of the bevel gearing-wheel D, the bevel pinion E, the counter-shaft

F, and the pulley G. The eccentric B admits of the shaft carrying the crushing head or truncated cone C approaching to and receding from the shell H. Inside the latter the concaves I are fixed, by which means the cone impinges the rock against the concaves. The material to be treated is

FIG. 107.-Gates' gyratory stone-crusher (sectional elevation).

dumped into the feeding hopper K, and after being broken leaves the machine by the shoot L to be elevated to the sorting sheds or screened under the foundation on which the machine rests. In the former case the whole of the broken material, of varying sizes, is distributed by conveyors into separate bins for immediate removal, or stored for subsequent use.

The foundation for this type of crusher may be of timber, mason work, or concrete, to which the bottom plate M is fixed. This class of crusher is made in different sizes, and is calculated to break from 2 tons per hour with a No. O machine, requiring 4 h.-p. (indicated) to drive it, up to No. 8, which is intended to break, according to the nature of the rock, from 100 to 150 tons per hour, requiring an engine with an indicated h.-p. of from 100 to 150 to drive it, including the screening and elevating arrangements.

The price of the No. O machine is £75 and that of the No. 8 £1400, which figures do not include the cost of the necessary foundations. This stone-crusher has not, unlike most other American inventions, been exten