was wholly carried out upon one side of the piston, notwithstanding there is a loss by throwing away the steam before the completion of the stroke, a loss as much upon the one side as the other. Though these two losses, when added together, may not amount to so much as if the whole time was taken from one side of the piston, the object of such an arrangement is merely that the vertex of the crank's action (being the slowest of the pistons) should divide the time necessary for the evacuation of the cylinder, and thus be the gainer of the double advantage in the peculiar situation of the engine at that time. By this practice, when properly managed, I do consider they have been the gainers of from half, to one pound better mean exhaustion, and that the best engines now, when working under their best arrangement in this respect, have not more than what is equal to 2lbs difference between the extreme condenser vacuum, and the main cylinder exhaustion; that is to say, they have but IIb upon the exhaustion side, and an equal loss upon the other, whereas, if they did not open the eduction valve till the end of the stroke, they would have had 24 tbs, or 3 lbs difference. So that I now consider that I have only 2lbs of difference to save by my engine, independent of the better vacuum that I can procure by the complete evacuation of my condenser every half stroke, at the very lowest amounting to half-a-pound, giving me 24lbs more effective power upon every square inch of my working piston from the same quantity of steam. But at the same time, when this extra quantity of power is not wanted, by carrying out the expansive principle to lessen it, I shall increase the duty in a much greater proportion.

Independently of this saving, the speed of my piston may be increased without any loss, to 400 feet per minute, thus diminishing weight, and saving first cost of the engine.

From the best authorities I find that the Great Western's engines, which "S." speaks of, when working at 600borse power, (actual,) consume about 4lbs of coals per horse power per hour, which is lifting, in the Cornish phraseology, about 41,000,000 for a bushel of coals of 94lbs; now this is only one

third of the duty of the best Cornish lifting engines. What can account for the whole of this difference ?

else

Upon that part of my paper respecting the compression of steam in the Cornish engines, "S." questions whether the pump-rods are of sufficient weight to balance the water, with or without weights. This does not affect the question. The engines referred to by Mr. Parkes and myself are those whose pumping stroke is effected by the weight of the pump-rods in "bob," therefore it seems clear that these rods, &c., must be equal to overcome the friction of engine and pumps, the weight of the column of water, &c. If so, I have said that when once in motion they would acquire a certain momentum, and, unless by some means brought gradually to rest, would strike the top of the cylinder, or where, to the injury of the machine. I merely meant to show that it is possible to shut in steam, as a cushion to bring the piston to rest, in such manner that, in some cases, it would be so compressed by the momentum as to exclude any possibility of percussive action by the next incoming steam. Such I know to have been the case to a considerable extent in some engines I have seen; but when it was so to any amount, the piston did not stop at the top of its stroke, but returned directly, and stopped at the bottom. I do not think that all the fifty-one engines have the pause at the top, as suggested by "S." Some do not pause at top at all; but all those engines that have done the greatest duty, when doing that duty, did pause at top; and when they work quicker, and consequently do not pause so long, or not at all, their duty suffers immediately in proportion.

I find "S." had founded his calculations in his former paper upon 17.75, which is taking our barometer at much above its average state. I had not done so, and did not notice that "S." had, when I said I thought he had given too high a mean. I had considered it at 17.5, taking the barometer at 14.5lbs 29.67 inches of mercury, about an average state for us. His mode of calculation is sufficiently correct for all practical purposes.

=

I cannot agree with "S." in adding lb to my vapour in the condensing

cylinder, for my cylinder will not be warmer, or so warm as the condenser of a marine engine, as it will be surrounded with cold water, and if it were, it would have only vapour of the clasticity due to the temperature of the water lying in it, which would be 93°b altogether, instead of, as "S." says, 1+3=14lbs.

At the same time I will admit that "S." has, perhaps, given me more than I may find in the 4lbs, so that his conclusion may not be far from the truth.

I beg also to thank "A. M." for his diagrams, which may be valuable to me. Will he oblige me with the following data, the first in particular, for at present both are useless.

The consumption of fuel per horse power per hour-the steam pressurewhen cut off-the height of the baromcter-the mean and extreme state of

the condenser vacuum as shown by the gauge at the time-the temperature of the condensement-the number of feet the piston was travelling per minute, and the mean steam pressure as shown by the indicator upon the same card as the exhaustion was taken from.

The second diagram, and all diagrams should be accompanied with the same particulars, or they can be no guide for science to determine from. If not noted by "A. M." he will perhaps be good enough to take others in this way. No. 2 allows me sufficient gain, to make my engine valuable, therefore I will not touch upon that at present, but am, Sir,

Yours respectfully, JAMES PILBROW. Tottenham Green, October 11, 1841.

PADDLE-WHEELS AND SCREW PRO-
PELLERS.

Sir,-Some time since there was a long controversy carried on in your Magazine, on the comparative merits of the paddle-wheel and screw propeller, and, as the question was never satisfactorily settled, allow me to suggest a method which would probably satisfy all parties. Let any two boats be taken, one with the paddle-wheel,

and the other with the screw. Let each engine make 1,000 strokes, and the distance be measured-this would be

LIFE AND LABOURS OF TELFORD.
NO. XI.

[Continued from vol. xxxiv, page 439.] The Nene Outfail, and North Level Drainage.

In the more advanced period of his professional career, Telford was actively employed in improving the drainage

of the immense fens on the eastern coast of England, an undertaking which from its extent, and the effects produced, may be looked upon as of national importance. The whole district of the fens, reckoning from Cambridge on the south, to a line drawn from Lincoln to Wainfleet on the north, is not less than sixty miles in length, by from twenty to thirty in breadth, occupying an area of five hundred and thirty square miles, or three hundred and forty thousand acres; but the operations conducted by Telford were chiefly designed for the amelioration of the more northerly division of this large and naturally fertile tract of country.

The new outfall for the waters of the river Nene was originally projected, in 1814, by the late Mr. John Rennie, and executed under the joint superintendence of himself and Telford; but the plan for the drainage of the "North Level” was due entirely to the latter. The works of both were carried on simultaneously -so complete was the confidence of Telford in the ultimate perfect success of the plan for the new outfall, though less practical engineers were full of doubt as to the result of a scheme which was then perfectly novel, and in no locality more so than in the district of the fens, where, reasoning à priori, the arts of drainage should be best understood. His confidence, however, appeared to have been well founded. The

river Nene now reaches the sea by a direct channel, over which a convenient bridge, connected with a good line of road, effects an always practicable communication between the counties of Norfolk and Lincoln, or, it may be said, between the eastern peninsula of England, and the whole of the midland counties. Previously, the only mode of transit was by boats, across a wide and dangerous estuary, passable only at certain times of the tide, and then at the utmost peril to life and property, in token of which it may be remembered that it was on this very spot King John sustained that heavy loss of all his cherished treasures, which, according to most historians, shortly after brought him to his death-bed. Similar, and even worse disasters have been common up to our own days, but now the passage is as safe and easy as the crossing of London Bridge. Besides this advantage, the improvement has enabled the land owners to embank 1,500 acres of land from the sea, all of which now bear luxuriant crops of grain; while 2,000 acres more are already fit for enclosure, and 400 more rapidly becoming so.

Nor are the effects on the navigation of the river less beneficial. There is now a safe and daily communication between Wisbeach and the sea, at all times of tide and in all weathers, in place of the old and peculiarly tedious navigation, which was practicable only at spring tides, and with a fair wind, and then only for vessels of 60 tons. Ships of 400 tons now reach Sutton Wash at spring tides, and might even get up to Wisbeach itself, but for want of the comparatively trifling further improvements, which would suffice to make that town the emporium of Cambridge, Norfolk, and Lincoln shires; as it is, its trade, which before the new outfall amounted to 50,000 tons, has, since its opening, more than doubled, baving reached to 108,000 tons per

annum.

But the most important result of all is, that the water in the new channel ebbs 10 feet lower than it did in the old one, immediately opposite to the South Holland and North Level sluices, which are the outlets for the drainage of no less than a hundred thousand acres of fen land, lying

between the rivers Nene and Welland. The consequences of this simple fact, unimportant as it might be in a less level country, are, that the means of natural and unassisted drainage are afforded to immense tracts of flat and fertile land, which, under the old system, were obliged to be cleared of their water by a most complicated, ineffective, and highly expensive arrangement of windmills, and (of late years) steam-engines. In nothing, perhaps, did the genius of Telford shine more brightly than in the complete revolution he effected in the whole system of fen drainage; and probably few of the stupendous works he superintended, will be found in the end to have been productive of greater national and individual benefit than those he carried into effect in the North Bedford Level.

In cutting an artificial canal, the engineer has every thing, as it were, within his own control; the means of overcoming his difficulties are before his eyes, and, with the aid of labour, within his reach. It is widely different in the case of natural rivers, and the subject, interesting as it is, has been strangely neglected until a very recent period. The reports of Smeaton and others, within even twenty years' date, show, that in such cases as that of the Nene, it was always recommended so to arrange the sluices as to shut out the waters of the sea, and prevent the tide from entering. So far all was well, but it seems to have been forgotten that while the sea was kept out, the land waters were also kept in, and thus the general level of the channel kept permanently higher. The error, in all probability, arose from the fact, that Dutch engineers have, generally, in past times, been employed in the drainage of the fens, as having had, beyond all contradiction, the largest experience in this line of any people on the face of the earth. Vermuyden and his companions naturally enough employed the same means which had been found effectual in their own country, and accordingly dammed up the mouths of every outfall with immense sluices to exclude the tide. This plan answers exceedingly well in Holland, where the rivers are no more than large drains, to take off the surface water from their own localities alone,