SHEET VIII.-EXPERIMENTS WITH SCREW-THREADS AND NUTS.

In my last report I gave an account of some experiments showing that the standard sizes of screw threads are much too coarse, and that there is a great waste of metal in bolt heads and nuts. These results showed that for a 14-inch bolt twelve threads to the inch gave nearly 20 per cent. greater strength than six threads, the standard number, and I stated that "it is quite probable that even finer threads than those tested, say sixteen or eighteen to the inch, would give better results."

As it requires over 4,000 of these bolts for the lock-gates, I have recently tested the matter still further, and find as predicted that 18 threads to the inch gives decidedly greater strength than 12 threads, and for some purposes it might be desirable to go to that extent in the reform, but other considerations besides strength will probably limit the number to about double the number required by the present tables.

In these latest tests three pairs of bolts were made, one pair having 6 threads one 12 threads, and one pair 18 threads to the inch, but in all other respects the bolts were as nearly alike as it was possible to make them. They were turned from bar-iron, 19 by 2 inches square (see Fig. 5), so that no forging was required.

The results were still more favorable for the fine thread, for when broken in a hydrostatic press not a single nut showed signs of weakness, and the bolts with 18 threads to the inch showed unmistakably that they were stronger than the others, although they finally yielded by pulling out of the nut-not by stripping the threads, as we generally understand it, but by actually drawing down the size of the bolt until the greater part of the threads were disengaged, as can be seen from the speci

mens.

In Fig. 4 I have plotted the strains and corresponding stretch of the 6 bolts, the dotted line showing the probable relation between the stretch and strain upon each bolt while under tension, and for comparison therewith a diagram enlarged from an autographic sheet (similar to an indicator diagram), taken in 1870, which is thought to be the first attempt ever made to get a continuous record of the behavior of iron under varying strains.

The standard bolts broke at an average strain of

Those with 12 threads at

Those with 18 threads yielded at.

Pounds.

76,655

92,991

94, 248

It was noticed that the latter were just on the point of breaking when the nut pulled off.

In Fig. 3 I have suggested a new standard for screw-threads in accordance with the indications of these experiments. That it would be a great improvement on the present standard I have no doubt, and I have endeavored to fix upon the simplest numbers for each size of bolt that will give about double the present standard number of threads to the inch, and avoid fractional numbers as far as possible.

Figs. 1 and 2 (from my report of 1883) are simply given to show the important advantage of having the strength of all parts of a structure proportional to the strains they are to bear. As the strength of a beam can be more than doubled by cutting away surplus material, as shown by these experiments, so bolts, nuts, and screwthreads can be made lighter, stronger, and cheaper by simply adopting proper proportions, instead of having one part two or three times stronger than another, as at present practiced.

BB 2.

IMPROVEMENT OF CUMBERLAND RIVER, TENNESSEE AND KENTUCKY.

The States of Tennessee and Kentucky made appropriations for the survey and improvement of the Cumberland River as early as 1830, and in 1846 a charter was granted by the former State to build locks and dams upon the river below Nashville.

In 1870 Congress authorized a survey of the Cumberland, and in 1875 a resurvey of that portion embracing Smith's Shoals. The present plan of improvement was based upon these surveys. In 1879 a survey was made of the Falls of the Cumberland, and in 1880 a reconnaissance of the Upper Cumberland. Another survey of the Cumberland River at Smith's Shoals, Kentucky, was made in 1881, to ascertain the practicability and cost of a canal, with locks and dams, from the head to the foot of said shoals, and a survey in 1883 from Point Burnside to Nashville, with a view to placing locks and dams on the Cumberland River between these points.

The Cumberland River has been divided into a number of different sections in the various river and harbor acts, and these sections will be considered in their order.

The obstructions to be overcome in the different sections are similar in character, consisting of rock-reefs, gravel-bars, snags, bowlders, and overhanging trees, and on that portion of river above the Jellico mainly of fish-traps and mill-dams.

The method of improvement followed has been to blast a channel through the rock reefs, remove gravel-bars and bowlders, build riprapdams where a contraction of the water-way is required to secure additional depth, and to remove snags and overhanging trees.

The Cumberland is navigable for all steamboats which ply upon it for six months in the year from Nashville to the mouth of the river, a distance of 192 miles, from six to eight months for boats drawing 3 feet and less, and the entire year for boats of 16 inches draught. Above Nashville the river is navigable to Point Burnside (the Cincinnati Southern Railway crossing), a distance of 327 miles, from four to six months each year for steamboats drawing 3 feet or less, and from two to three months for larger boats. From Nashville to Burksville, 238 miles above, the river is navigable for steamers of 3 feet draught for from five to seven months, and from three to five months for the larger boats. From Nashville to Carthage, 118 miles above, the river is navigable from six to eight months for steamers of 30 inches draught, and from four to five months for larger boats.

The work done upon the Cumberland, though not yet completed, has already had the effect of extending the duration of navigation both above and below Nashville.

The estimates for the improvement of the Cumberland River have been based on the idea that the appropriations would be made for the entire work in a reasonable time, say four or five years; but in the case of the river below Nashville the annual amounts have been so small that although fifteen years have elapsed since General Weitzel made his estimate, the full amount has not yet been appropriated; and above Nashville only $50,000 have been appropriated for a system of locks and dams estimated to cost over $4,000,000, at which rate it would take eighty years to complete the work. Of course it would be impossible to carry on work with proper economy under such conditions, and I would respectfully urge that Congress may give the subject its due

consideration, and if the more radical system of improving this river under the existing projects is decided to be advisable in view of the necessities of commerce, an appropriation of at least one-tenth of the estimate for completing the work may be made.

The shipments made on the Cumberland from July 1, 1884, to June 1, 1885, are as follows:

Active operations were resumed on this section August 7, 1884, and continued until November, when high water caused a suspension until June, 1885, when work was resumed at Line Island, and is now in progress. C. A. Turrill, assistant engineer, is in local charge.

The following is a statement of the work done at the localities named: At Race Track Shoals and Ingram Shoals a dam was repaired at each obstruction; 236 cubic yards of rock were quarried and put in dams. At Little River Shoals a dam was extended by adding 314 cubic yards of rock; 4,530 cubic yards of gravel were taken from the channel at Sycamore Bar, Half Pone Bar, Medlock's Bar, Seven Mile Ferry, Palmyra Island, Well's Island, Elk Creek Shoals, Line Island, and Ingram Shoals. Near the mouth of river (Cumberland Island) one brush and stone dam was built; 34 piles were driven, 932 cords of brush and 875 cubic yards of stone were used; 2,595 cubic yards of gravel were also removed at this point.

In addition to this a snag-boat descended the river from Nashville to the mouth of the river, removing 128 snags and 14 cubic yards of rock en route.

As a result of the season's work a channel of at least 2 feet at low water, and 80 feet wide, was secured at the Cumberland Island Bar, at the mouth of the river, and a small packet steamer made continuous trips over the bar during the low-water season of 1884.

The amount asked for ($50,000) can be profitably expended in clearing the channel of surface obstructions, in continuing work of improving the most dangerous shoals below Nashville, and in improving the channel from the mouth of the river to deep water in the Ohio by building brush and stone dams, dredging, &c.