very small indeed. In the former case it will depend entirely on the weather, and as the leading hands are paid full time even during delays caused by wet weather or otherwise, it is necessary to allow a sum to cover this loss. The experience of the author is that an amount equal to 0·32d. per ton will be ample to meet such a contingency. These items summarized represent the actual cost of preparing macadam delivered into carts or wagons under ordinary conditions, and are as follows:

:

In the event, however, of a quantity of chips and gravel
being utilized in rolling or for other operations, a sum
equal to say 15 per cent. of the value of the total amount
of screenings should be deducted, amounting to

Total cost after deduction, .

0.50

10.06

The cost of labour and the fuel for the engines have a bearing on the total cost per ton of macadam broken, and may vary considerably in different counties and districts. The author has on many occasions accomplished this work for 9d. per ton (but the inclusive cost has generally been 101d.) by arranging the work to be carried out during the most suitable months of the year. If the working expenses incurred be calculated on the total amount of rock broken each day, then the cost, taking the quantity as 72 tons, would be 8.32d. per ton.

On referring to p. 181 it will be seen that the comparative cost of hand-broken macadam is 1s. 3d. to 2s. 6d. per cubic yard, or 1s. 1d. and 2s. 3d. per ton respectively. The average, however, may be taken at 1s. 9d. per ton, which represents the price paid for this class of work previous to the introduction of stone-breaking machinery into the district of which the author has charge; but it may be incidentally remarked that the price for hand-broken macadam is now much higher. The difference in the cost of production between hand and machine-made macadam is therefore, taking 11d. per ton for the latter, nearly 48 per cent. in favour of macadam produced by a stone breaking machine. It may be mentioned here, although the matter will be described fully in the chapter on "Rolling," that the macadam produced by a breaking machine, being properly screened, makes a much superior road surface compared to that which can be made with unscreened hand-broken metal.

If hand-broken metal be screened so as to improve its wearing capacity, then the saving in favour of macadam produced by machinery will be still greater.

268. The requirements of a district or county in regard to the quantity of macadam required for maintenance will depend in a great measure on the amount of traffic, the quality of the material used, and the mileage of roads to be repaired each year. When the quantity does not exceed 8000 or 10,000 tons each year, one stone-breaker of the construction and size described will accomplish this in from seven to eight months, allowing for the time lost by occasional wet weather and other causes. If the quantity of metalling required each year exceeds these figures, it will be more economical to employ two or more machines, so that the total quantity necessary for repairs may be broken during the summer months. One stone-breaking machine working during the whole year in different quarries will be capable of producing, in addition to the 8000 tons already mentioned, a quantity during the other four or five months of the year of from 3500 to 4400 tons, or a total for the whole season of from 12,400 to 13,500 tons of macadam.

The cost of production during the winter months will be increased considerably, and in many instances may exceed the cost of breaking the material by hand.

This does not, however, apply when fixed machinery is employed, where the work is carried on during the whole of the year, although the cost is sometimes increased by working on a short day, or even on full time at that period of the year when using the electric or other light. Under these circumstances it is usual to erect a covering over all the stonebreaking plant in order to afford shelter for the men engaged, and so admit of the work proceeding without interruption. This also protects the machinery during unfavourable weather.

269. Amount of work performed by working parts, and Cost of Renewals.-The wearing parts, namely the jaws, wedges, toggles, and toggle bearings, lower and top revolving screens, elevator belt and buckets, etc., are easily renewed; their life of usefulness will depend on the amount of work performed, the nature of the rock broken, and on the care with which the different parts of the machine are attended to.

Taking an average over many years the more important parts mentioned will, depending on the quality of the material used in their manufacture, perform approximately the following work before the necessity arises for renewing them.

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.

[To face p. 204.

UNIV

OF

MICH