P.—s

3

mation is corroborated by Mr. Hamilton Smith, jun., of San Francisco, in answer to a letter that the Manager of the Kumara Water-race'sent, asking for information on the different systems of pavement for ground-sluices. He says : "In our North Bloomfield Tunnel, which is some 10,000 feet long, we have a grade of 4-J to 100. At the upper end we have a sluice 6 feet wide, and 2-J- or 3 feet deep above paving, and about* 2,500 feet long; the upper 1,000 feet is paved with wooden blocks 13 inches deep, which required replacing every fortnight; the lower 1,500 feet is paved with boulders of very hard rock, generally black quartz set in 18 inches in depth, and which wears four to six months. The rest of the tunnel, some 7,500 feet, the water and tailings run on the natural floor of the tunnel, which was driven through hard metamorphic state. The tunnel has been in use eight years, and in this time has worn down about 2|- to 3 feet. The wooden paving is much more readily and quickly moved than stone paving, and hence is used at the head, where we wish to clean up every fortnight. Those clean-ups only occupy about ten hours' time, say twelve hours from the time that the water is turned off until it is turned on again : to do this some forty or sixty men are employed. In this tunnel we run constantly day and night about 4,000 cubic feet of water per minute ; occasionally, however, we _use 6,000 cubic feet of water per minute. In this mine it requires from 18 to 25 cubic feet of water to wash and transport 1 cubic foot of gravel measured in the bank. The gravel contains many large boulders, and the sluice will transport readily a boulder weighing 500 lb. or more. Cast-iron paving has been used, but proved too costly. In a few instances along the line of the Central Pacific Eailroad all worn-out rails have been used, weighing about 56 lb. per yard, at a cost of $35 per ton, which is equal to about £7 6s ; the rails being laid lengthways in the sluice." The experience of mining companies in California show that wooden block pavement is the most expensive system of paving sluices ; but it is used because it is readily replaced when the boxes are cleaned out, and therefore seems to be more used for saving the.gold-at the head of the sluice. The wear and tear, however, seems to coincide approximately with the wear and tear in the Kumara Sludge-channel, i.e., the wooden blocks had to be replaced once a fortnight in the Californian sluice, where they work day and night continuously ; or, say, twelve days of twenty-four hours each, equal to 288 hours, that the blocks last apparently without turning ; and the wooden blocks in the Kumara Channel lasts approximately about three months, working on an average thirteen and a half days of eight hours each per month. This makes 324 hours, and shows that the wear and tear are greater in the former sluice. Having compared the wear and tear of the wooden block pavement with mining companies who have had larger experience in hydraulic operations, the same basis may be assumed for the wear and tear of stone pavement. It appears to last from four to six months in the Californian sluices, and, to take the minimum period as a basis, then four months, or, say, seventeen weeks, equal to 2,448 hours, that the stone paving lasts. This shows that stone paving would last in the Kumara Sludge-channel for twelve months, working steadily for eight hours each day ; and on this basis the monthly cost of maintenance ought not to exceed, say, £183 per month, which is arrived at in the following manner : —

By adopting stone-paving it will reduce the carrying capacity of the channel to some extent; probably it will only take forty-eight heads of water, instead of fifty-four which it carries with the wooden pavement; and therefore calculations ought to be based on this carrying capacity to get at the approximate revenue. This quantity of water would employ about one hundred men, and the income from those, at the present charge for the use of the channel, assuming them to use it twentyone days each month, would be £175, or £8 less than the cost of maintenance. Before going into the cost of receipts and expenditure if the channel was widened, it will be necessary to first determine the width. This can be done from the carrying capacity of the supplyraces and tunnel. These have lately been widened in order to carry 100 heads of water, and, when the present works are completed for the storage of a large body of water, I think that at least seventy-five heads can be safely calculated on, unless in unexceptionally dry weather. If the channel is widened at all it therefore ought to have a carrying capacity equal to seventy-five heads at the least, which is 4,500 cubic feet of water per minute. The North Bloomfield Company's sluice, before referred to, has to carry a similar quantity of water, and has somewhat of a similar grade. Their sluice has a fall or grade of about lin 22-J-, and the" Kumara Channel 1 in 26. Mr. Gow, the manager of the Kumara and Waimea water-races, thinks that 4 feet 6 inches would be wide enough, but this is at variance with the experience of mining companies who have carried on large hydraulic operations for a number of years in California, who show that it ought to be at least 6 feet wide. In order to widen the channel to 6 feet, one side of the tunnel for 38 chains would require new props 18 inches longer than the present ones. To do this, and widen the channel, I estimate would cost £3,500; but this widening could be done in such a manner that it would not stop the miners using the channel for more than about ten days. The cost of maintenance, if the channel was widened, would be about as follows :—

Description. Unit. Quantity. Rate. Amount. 3tone pavement ... )ne man to look after channel light (candles) incidentals, lump sum lin. ch. days lb. 85 313 8,854 £16 12s. lOd. £ s. d. 1,360 0 0 187 16 0 368 18 4 277 4 0 Total, per annum £2,193 18 4