CORRESPONDENCE.

Lyttelton Times, Volume XXI, Issue 1233, 10 May 1864, Page 5

CORRESPONDENCE.

LYTTELTON lIARBOR WORKS. TO Til 10 KIHTOIt OK Til 10 I,YTTUI.TON TIMKH. Slit, —I can assure you that Ido not wish to appear obnoxious to the imputation of a sort of 44 after me the deluge" impertinence, bnt I must offer a few remarks upon the subject of the communication No. 27, Lyttelton Wharf Commission, in England. The impression seems to be that the submarine deposit before hard rock is arrived at in the Port of Lyttelton is much too soft to admit of screw piles being used. When the prickings were taken, of course that instrument was naturally used which would prick until the rock was reached, with the greatest ease. Now, that the sludge should be very deep and very soft ia tliat part of Lyttelton known as Erskine Bay is the most natural and proper thing in the world— that recess must receive the deposited silt of all the ebb ; its comparatively still waters invite and determine this deposit. But what a notable feature of economy does not this depth of sludge assume, when it is remembered that we propose to dredge out this area so as to increase the depth of water. Beneath this sludge the ground has proved retentive enough, since the vessels frequenting the Port of Lyttelton have learnt, as they did at an early date in our history, by short, sharp and paiuful experience, that their ground tackle must be about 20 per cent, heavier than ordinary; and on the score of tenacity there is little reason to abuse the ooze, mud, and clay, that lies on the top of the rock and underneath the sludge. No wonder the sharp instrument seemed to penetrate by its own weight ; but every man who has been accustomed to 44 the lead " knows how, in a far stiffer bottom than this sludge, the lead will 44 suck." It appears, then, to be simply a question how large the spiral or helical flange or screw shall be, to give the proper amount of resistance to our piles and mooring screws, since the holding power depends on the area of the disc. The dimension may be extended to meet any case, regulated only by the facility of manufacture, and the power available for forcing it into the ground. In the 44 Practical Railway Engineer," by Dempsey, plate xxxiii., the sorts of screws then (1855), in use may be seen. Since that time, Johnston, the sole lessee of the first patent, has both improved the original invention and reduced the cost.

I am in a position to state that dredging machines are to be obtained of Sydney construction, which have received very high encomiums iu the press for their great mechanical ingenuity of structure, economical adaptation of steam power, and great results, eifected at a low cost. The Government of Queensland continue to approve those constructed for that colony, and find that their per- I formances are most satisfactory and encouraging. I cannot therefore see why we should yet delay an order for a steam dredging machine, seeing that this deep top surface of sludge cannot be too soon got rid of, and a place for its deposit selected. What New Zealand wants is the presence, in an official capacity, of a talented practical hydraulic engineer, the range of whose experience must embrace the whole question of lighthouses andthenice adjustment of all their machinery; for no system of universal economy and perfectability can ever be arrived at without such an engineer as Mr. Balfour, now in the service of the province of Otago, who, having served under Allan Stevenson, perfectly understands the physical laws to which attention must be given. . , , Let me make the following quotation from Weale s Rudimentary Treatise, No. 15, page 164, as most pertinent to this subject:— "Finally, it cannot be too often repeated that before undertaking any works which may interfere with natural causes so complicated and so numerous as those which affect all river or sea engineering, the most cautious, patient, and at the same time comprehensive view of the whole subject, must be taken. In the whole of professional practice the questions connected with hydraulic engineering are the most abstruse, and require the assistance of all the collateral sciences, and the profoundest acquaintance with the great laws of nature affecting the configuration of the globe. " Empirical knowledge is of little service, and the local engineer, if devoid entirely of theory, is as likely to fall into serious error as to hit upon the truth by trusting alone to his limited experience. And it is upon this score that we may account for the fearful waste of money, and the ruin of many ports which have been entrusted to the care of those whom it is too much the fashion to admire in our own country, under the specious title of " practical men." Extremes are dangerous in all things, and it must be understood that whilst thus advocating the necessity for wider and more theoretically scientific examination of the circumstances affecting the sites of any proposed hydraulic works, due appreciation is accorded to the value of local experience. Theory, practice, and science must mutually be brought to bear in all such cases, and the more of all of them the better. No one alone will suffice. The only axioms which can be safely laid down are: " 1. That no change should be introduced into the natural regime of either rivers or sea, until absolutely necessary. " 2. That all our efforts should be directed to bending the powers of nature, so as to ensure their co-operation in bringing about the state of things we desire to secure. The motto of the hydraulic Engineer should be that placed by Leupold at the head of his 4 Tlieatrum Machinarum Hydraulicum.'— Artis est naturam imitare." This extract appears to contain such valuable truth that we cannot afford, as pupils in the schools of political and mercantile economy, to ignore its teaching for a moment. I am, Sir, your obedient servant, ROBERT GREAVES.