It was perfectly comprehended by Bourgois, that in order to make the influence of a curved generatrix sensible, it would be necessary that the screw have a very considerable slip; its surface during the experiments, was therefore sufficiently reduced to satisfy this condition. To fully test this influence, the screw was first tried, propelling with the convex face, then tried propelling with the concave face, and lastly tried after the flexure had been taken out of the generatrix, that is, after the generatrix had been made straight.

Allowing for unavoidable errors of observation, dimensions, &c., in experiments of this nature, it will be observed, that sensibly the same result was obtained, propelling with either face of the curved generatrix and the straight generatrix, showing that the employment of a curved generatrix was at least useless, even with the exaggerated slip of 50 per cent.; 30 per cent. being the maximum in practice.

After reviewing some experiments carefully made on other screws, for the purpose of determining the effect of a curved generatrix, Bourgois remarks, which I translate as follows:

"In the second place, if we observe helicoidal surfaces with curved generatrices, or what amounts to the same thing, generated by a straight line inclined on the axis, we perceive the liquid threads do not rest on the same helicoidal thread. As the periphery of the screw is approached, the helicoidal thread inclines its propelling surface towards the axis, and the liquid thread keeping at rest in the normal plane, inclines itself slightly towards the centre; there, on the contrary and for the same reason, the liquid threads tend to remove themselves from the axis, but being endowed with less momentum than the first, there results a flowing of water towards the centre of the screw with so much the more abundance as the curvature of the generatrix is greater. This is the only notable effect resulting from the employment of a curved generatrix; and there is nothing to prove that effect favorable.

The experiments made on screws B7 and B 8,* show, on the contrary, an increase of slip of 04-7, proving that the employment of curved generatrices directs the water towards the axis in consequence of their obliquity.

The experiments on screws, and, under similar conditions, gave sensibly the same result, either the water was pushed towards the axis, or it was deflected out towards the periphery.f

Finally, in passing from screw 2 to 2,, the slip increased 07.5." The experiment of Sabloukoff, on a screw turned in air, and having the phenomena made visible by smoke, also corroborates the above. It was found that, after a high rotary speed had been given to the screw, the smoke being then let on at its anterior extremity, at any point near its periphery, was drawn towards the screw, and carried towards the other extremity. When let on at its anterior extremity, near the axis, the smoke coursed along parallel to the axis, wi hout any appearance of having a circular movement, and which was the same in the first case; spreading out from the axis, which should have been the case had the rotation of the screw been able to give a sensible centrifugal force to the smoke.

But even supposing, (which we have seen is far from being the case,) that the centrifugal force co municated to the particles of water in contact with the propelling surface by its rotary movement, were great enough to produce so sensible a result as a vacuum at the axis of the screw equal to a diameter one-fourth the diameter of the screw, and supposing the use of a curved generatrix to wholly obviate this, or restore solid water in the place of the vacuum; even then the employment of a curved generatrix would be useless as far as the reduction of the slip of the screw is concerned, and this fact also depends on the carefully conducted experiments of Bourgois. In those experiments there were tried two screws, exactly alike, excepting that the one had a projected area at right angles to axis, of 187-86, while the other had a similar area of 182 59, the reduction being made by cutting out the surface immediately around the axis. The diameter of the screws was 15.752, and the diameter of the cut out part of the last screw was 3.938. The slip of the first was 35.2 per cent.; of the last, 32-6 per cent. Similar experiments on two other screws, differing from the above in pitch only, gave with the full screw a slin of 26.9 per cent., with the cut out screw 24.4 per cent. On these experiments Bourgois remarks, which I translate as follows:

"The difference (be seen the slips being little enough to be attributed to irregularities of construction or slight errors in the observations, Which were precisely alike, and formed with the generatrix tangent to an inner cylinder, which is virtually a curved ge oratrix. B 7 propelled with the obtuse or convex face, and gave a slip of 26.1 per cent. B8 propelled with the acute or concave face, and gave a slip of 30-8 per cent., or 4-7 cent. more.—B F I. †Screws +3 and 44' were precisely alike, and formed with the generatrix tangent to an inner cylinder. Screw 3 propelled with the obtuse or convex face, and gave a slip of 32.8 per cent. Screw 4 propelled with the acute or concave face, and gave a slip of 33 per cent.-B F I.

Both screws being precisely alike, except that screw 2 had a straight generatrix; and gave a ship of 34-5 per cent., while screw 2, had a curved generatrix, and gave a slip of 42 per cent.-B FL.

nothing can be concluded from it, except that a hollowing out, of which the diameter is equal to the fourth part of the exterior diameter of the screw, has no influence on the slip."

Believing that sufficient has been written to put this subject in a true light, there only remains to notice a few of Mr. Nystrom's statements. And first he says:

"The water between the blades is acted upon at the same moment by two forces, the one being the propulsive force resulting from the oblique action of the revolving blades, and the other being the centrifugal force generated by their rotation, the first force tending to force the water backwards in directions parallel to the axis of the propeller." This is erroneous, the water forced backwards by the blades of the screw is in directions at right angles to the surface of those blades; no fact in hydrostatics being better established than that fluids are always pressed in directions at right angles to the pressing surface.

Mr. Nystrom gives the slip of screw propellers for ocean steamers at 50 per cent. This is not the fact, 30 per cent. being the maximum, and any screw giving a greater slip, should be immediately (and would be) changed for another of better proportions, giving the proper slip of about 25 per cent., which gives the minimum of the losses of labor by slip and friction of the screw surface on the water.

Mr. Nystrom also recommends the employment of bands for strengthening the blades, to be applied at the periphery of the screw. Now the propriety of using bands at all results from the fact that the water acted upon by the blade in its lowest position, offers a greater resistance on account of its greater depth than the water acted upon by the blade in its highest position, but the blade must possess strength enough to withstand the greater resistance; if, therefore, the excess of strength which it would have when subjected to the less resistance, could be applied to it when subjected to the greater resistance, a less strength of blade would answer. In the common paddle wheel, this is effected by bands or rings uniting the extremities and intermediate points of the arms or radii bearing the paddles, and Mr. Nystrom adopts the same system for his screw propeller, without, however, recognising the important difference, that the reaction on the paddle-wheel arms is at right angles to those arms, and directly in the line of the uniting rings, while the same reaction on the blades of the screw being at right angles to the helicoidal surface of those blades, and that surface being oblique to the axis of the screw, and not in the same direction as in the paddle-wheel, the reaction would not be at right angles to the axis, and as the rings or bands are placed at right angles to the axis, would not be in the line of the bands, but oblique to them, so that the bands, instead of being subject to a strain in the direction of their length only, would be subject to an additional cross strain, and in proportion to the sines of the angle of obliquity of the blades. To resist this cross strain, the bands would have to be impracticably large; and if not made strong enough for this strain, they are useless. Furthermore, if used at all, the bands ought evidently not to be applied to the periphery of the blades, but to the centre of effort of the blades. They are, however, a totally unnecessary and worthless appendage to any submerged screw.

When the date of Bourgois' experiments are considered, so far antecedent to Nystrom's patent, it is really amazing that our patent office should have granted a patent without novelty; but as I apprehend no one will ever be found to use it, so the issuing of the patent will do no harm to any but the inventor, who suffers to the extent of the patent fees and cost of model, &c.

For the Journal of the Franklin Institute.

Sketch of the Invention of Parker's Water Wheel. By Z. PARKER. In the year 1825, Z. G. & A. Parker, brothers, were in partnership, erecting mills in Cashocton county, Ohio, on Wills Creek, a navigable stream, much subject to "back-water." Having started a flutter-wheel saw-mill in July of that year, it was found, from freshets occurring frequently, and the known character of the stream, that the wheel would be flooded, and the mill consequently inoperative a great portion of the time.

To remedy this disadvantage in a second saw-mill in the same building, it was contemplated to build a geared re-action mill, such as were common at the time. But these mills being expensive, and hard to keep in working condition, this arrangement was not very satisfactory. In the latter part of august, Austin Parker suggested the idea of avoiding gearing, and making a wheel that would run under water, by arranging a number of small re-action wheels vertically on a horizontal shaft, to occupy the same position as the flutter-wheel. After due consideration, this suggestion was approved, and in November, a pattern for cast-iron buckets for such a wheel was made by myself, and taken to a foundry in Zanesville to be cast. The castings were got to the mill in February, 1826; and in March or April, the construction of the wheel, consisting of six (two double and two single) small re-action wheels, 28 inches diameter, was commenced. In November, the arrangement was so far completed, as to admit the application of the water, as an experiment to enable us to judge of the power and speed of the wheel. The result indicated an entire failure, the wheel only making about 80 revolutions per minute, running free, when at least 180 were expected; and it was held with a slight resistance to its motion.

In consequence of this failure, the work on the new saw-mill was suspended, and our attention turned to the erection of a flouring and gristmill. For this, a common re-action wheel was made, which was put in operation in the fall of 1827. While at work at it, and before the gearing was attached, a small stream of water was admitted to pass through it, which was slowly turning it. The water, in consequence of the direction of its approach, fell into the wheel with a vertical or whirling motion, which was in the direction contrary to the motion of the wheel. A plank by accident fell into the penstock, and lodged in a position which changed the direction of the water, so as to cause it to enter with a circular motion, coinciding with the wheel's motion. The wheel was at once observed to start off with nearly double its previous speed. The plank was removed, when it again relaxed to its former motion; and by frequently trying the experiment with the plank in and out of the place where it

lodged, it was satisfactorily ascertained, that the wheel had nearly double the speed and power, when the water entered it revolving with it, as when in the contrary direction.

This discovery brought to light an error which had been committed, in the introduction of the water to the saw wheel, which was inadvertently made such as to cause it to enter whirling the wrong way; and the confident hope was then revived of yet making it successful. Accordingly, in the summer and fall of 1828, such alterations were made in the adductive passages as would cause the water to enter the wheel with a lively circular or helical motion, coinciding with the revolution of the wheel. On introducing the water, the success was found to be complete; and the same wheel, which with the water entering the wrong way, made only 80 revolutions per minute, now made 280, and apparently with great power. This trial was made in the latter part of October.

In November, with a view of an experiment in applying the principle more perfectly, a small re-action gig-wheel was made, (for backing the log-carriage of the same mill,) with such modifications in its form and proportions as were thought to be necessary to adapt it to the new mode of action of the water, to which the first helical sluice, in the form represented in the patent, was applied. Its speed and power were beyond our most sanguine expectations.

The saw-mill was got fully into operation in December, 1828, and such was the success, that with an aggregate orifice of 250 square inches, on a fall of 5 feet, the new wheel would saw 3000 feet of boards in a day; while the flutter-wheel, (which was considered a good one,) with a gate of supply of more than 450 square inches, would only saw 2000 feet. The alteration of the saw-wheel, and the planning and construction of the gig-wheel, were done almost entirely by myself, and under my directions.

The "new idea" of the "draft boxes" originated with Austin Parker, then in Portage County, Ohio, early in 1833; and he made a number of experiments to satisfy himself of the practicability of the principle, and applied them imperfectly and experimentally to several mills, and was making further experiments in their application at the time of his death. in October, 1831. Further investigations and experiments, and the final perfecting of this part of the invention, was done subsequently by myself.

The beautiful and interesting experiment which manifests the law of the centrifuge and equilibrium of a mass of water, or other fluid, revolving within a vertical cylinder, variously changed and repeated, so as certainly and fully to establish the truth of the deduced law, was made by me in Licking County, Ohio, in January, 1811, and a notice of it with a diagram, together with various statements, drawings, and models, relating to the invention, was sent to the Franklin Institute, in May of the same year.

An extensive set of experiments was also made at the same place in April, 1844, for the purpose of ascertaining the true value of the circular motion of the water, in its application to re-action wheels, and the superiority of the improved form of the wheel over the old; and a paper containing a tabulated statement of the results was presented to the Institute in the May or June following.

VOL. XXII.-THIRD SERIES.-No. 1.-JULY, 1851.

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