By means of the suspending apparatus above described, the lever is enabled to obey any force acting vertically on its longer arm, with the advantage of ample strength and stiffness, combined with the condition of a theoretical lever, in respect to the gravity of parts.

There are two modes of operating by which a bar of metal placed in the machine between b and b". ight be broken, so as to ascertain the tenacity.

The first is to apply the force of the screw S, to strain the bar in raising a weight W, suspended at any convenient point on the arm h of the lever; the second is to employ the screw only to regulate the height of that arm, and to restore it, when relieved of the weights, to the horizontal position, whenever the extension of the bar had allowed it to fall below that position.

The latter method was, with very few exceptions, adopted by the committee, both because it allowed of a more exact determination of the breaking weights by a small addition at a time, and because it rendered the effect of the friction constant in its kind, being al· ways in opposition to the gravitating force of the weight W, and subtractive in the calculation. In order to apply this mode of action without requiring correction for the stiffness of the cord r' and the friction of the pulley p', it was only necessary, after adjusting the weight C', to remove so much as would allow the arm h of the lever to descend upon the slightest jarring of the machine. The tenacity of the bar, and the friction at the fulcrum, were then the only resistances to the motion of the weight W.

The purpose of the tackle of pullies P, is to elevate the scale pan and weight after they have descended to the floor, in order, by turning the screw S, to counteract the elongation of the bar under trial, and again to commence operations with the descending motion of the arm h. The power of the operator is applied to the tackle by means of the windlass w, furnished with crank, ratchet wheel and click. The upper edge of the lever was graduated into parts, distant from each other just ten times the length of the

shorter arm.

By the aid of these several appendages, the machine allows the most gradual additions to be made to the divellent force applied to the specimens, breaking each with a descending movement, and consequently, rendering the friction definite in the direction of its influence, being, as before stated, always subtractive.

The very few cases in which the mode of operation rendered it additive, are particularly mentioned in the tables.

At the outer extremity of the lever, and in the prolongation of its upper edge, is placed a style z, serving as an index to the graduated arc a, which is divided into minutes of a degree. The point of the style is ten feet three inches from the axis of motion in the lever, and the length of the entire circumference which it would describe 772.8276.inches. Hence each degree is 2 24674 inches and each minute, as measured on the arc, is 0.35779 of an inch. The whole extent of the arc a is about 5°, the zero, or point of horizontal position being placed three degrees from the upper extremity. The chief use of this arc was to determine approximately the elas

ticity of the bars, and of the machine itself, as preliminary to that inquiry. The weights W, in the scale pan, (which, with its suspending chains, cross bars, &c., weighs 56 pounds,) were in every case applied on the lever, at the third mark, a distance from the axis of motion 30 times as great as that between the axes of the two bolts, or 30 x 2.914 87.42 inches.

=

Friction of the machine. The amount of friction of the machine already described for testing the tenacity of metals, was an object requiring particular investigation before anything more than a comparative value could be assigned to the results which were afforded by the experiments.

To determine this point, it was deemed advisable to ascertain, under various loads, what proportion the weight which was sustained by the machine, after it had been raised by the screw S, till the index stood at zero on the arc a, bore to that which, after the lever was relieved and then loaded again with the same weight, would cause it once more to descend to zero.

Between the heads b' and b" was placed a strong bar of iron one inch wide by three-fourths of an inch thick. Two methods were then pursued for the purposes of mutual verification.

I. A certain weight was placed in the pan suspended at h, and the screw S turned until, as before mentioned, it was raised to a level, so that z stood at zero on the graduated arc. The windlass w was then employed to raise the scale pan and entirely relieve the lever. On again restoring the weights, the index remained some minutes of a degree above 0, and an additional quantity of weight was necessary to bring it once more down to that level. As in this case the weight added served to increase the friction, it is manifest that the comparison of it with the whole weight, itself included, must be necessary in order to show the relation between the weight as first raised and that part of it which represented the friction of the machine. When the weight was raised by the screw, the bar which connected the heads b' b' must have sustained a strain composed of the weight raised, added to the friction of the machine; whereas, when the weight was let on by the windlass while the index was at some distance above 0, the bar sustained a strain represented by the weight borne, diminished by the friction.

II. The lever was caused to rest on a solid support near the extremity, the index being opposite to zero on the arc, and in that position the scale pan was loaded with the weight under which it was intended to try the friction. The screw S was then carefully turned to strain the bar and bring the loaded arm of the lever barely off of the support. The weights were next raised by the windlass, and the recoil of the machine raised the lever to a certain elevation, from which it was once more depressed by replacing the weights upon it, and adding such an amount as would just depress the arm to the level of its original support.

The first of the above methods gives the double, and the second the single friction of the machine. The following table exhibits the weights in the scale pan, the weight representing the friction when the first method was employed, the same for the second

method, and the ratio of the friction to the total weight sustained by the lever. A correction is required particularly at the higher pressures, on account of the increased elasticity in the machine under the added weights, which actually brought the index down to zero sooner than it would have arrived at it, by the simple effect of a strain upon the lever regarded as inflexible.

The machine was kept constantly well oiled, still a trifling difference may possibly have existed in regard to its condition at different times, but no influence of this sort was ever found sufficient to determine the rupture of a bar, after the weight had been taken up, the gudgeons newly oiled, and the same weight replaced which it had borne previous to that operation.

56

6.00

.050+

112

10.82

.051+

168

17.62

.052+

224

24.85

.055+

10

280

28.16

.050+

336

35.30

.053+

.047

374065E

.048

.045+

Mean .050

Total 44

Mean.050 5 Total 47

From the above table it appears that the second method gave results more nearly in accordance with each other than the first, but it will also be noticed that forty-four observations with the first give a mean value sensibly identical with that obtained from forty-seven experiments with the second method of trial above described. were hence led to adopt 5 per cent. on the weight as the effect of the friction of the machine. The bolts are of well polished steel and the lower bearing of cast iron, and the upper one, or the eyes of the straps "i", of wrought iron.

This relation of friction to pressure between these substances, as deduced from the experiments of the committee, will be found to correspond very nearly with that obtained by Mr. Wood, when ating on railway carriages.

sticity of the machine. In order to determine, in particular

cases, the amount of elasticity exhibited by the bars under trial, it became necessary to ascertain the elasticity of the machine, when loaded with different weights. Several series of trials were accordingly made, expressly with a view to this object.

Putting into the machine, in place of a bar to be broken, one which was intended not to yield sensibly to the strains applied, and not in any case to be permanently elongated by them, different weights were appended to the lever and allowed to remain until the latter had become stationary. They were then carefully raised by the windlass, and the lever allowed to rise by the recoil until' it became entirely free from strain. The number of degrees and minutes on the arc a, traversed by the index, was then noted, the weight replaced, and the trial repeated until it was ascertained that no error of observation had occurred.

Three series of operations were performed, each beginning with the lever 3° 30' above zero, when entirely unloaded, but fully in contact with its bearings; weights were then added by 56 pounds at a time, and the depression below 3° 30', produced by each addition, was noted. This was continued until weights had been added sufficient to bring the index to 0, which was effected with 11 weights of 56 pounds each.

If the lever had been entirely inflexible, the natural sine of the angle of elevation after being relieved might have been considered the measure of the compression of parts sustained by the frame, links, &c., under each weight; for as the shorter arm of the lever is only 2.914 inches in length, while the bar and connecting straps are more than five feet, the direction of the horizontal bar may be considered sensibly constant.

The following table contains the results of the experiments just described, together with those of another set in which the operations were in every respect similar, except that the weights were applied by 37 pounds at each time instead of 56 pounds. The natural sines are added, by comparing which with the respective compressing forces, it will be found that the law which, governs the elasticity of the machine is that the latter is proportionate to the fifth root of the cube of the compressing force.

47.2 .0136713

TABLE II.

Remarks.

.0116353 Comparing the first with the last experiment by the for

mula

616
37.5

x nat. sin. 211.5'

=

we get x 0.594

nat. sin. 40'

86.

168.

95.

187.5

100.

224.

112.7

262.5

120.

58. .0168707 The 3d and 16th, by a similar comparison, give x = 0.608 74.7 .0218149 The 4th and the 19th give x =

128.7

Another set of trials was made, loading the lever with weights by seven pounds at a time from 0 to 609 pounds; and from the results of this and the preceding series, a table was constructed, furnishing the column of elasticities of the machine for every observed depression under given weights when testing the elasticity of bars of iron. By deducting the elasticity due to the machine alone from that obtained by observation, we get the measure in minutes of a degree, of the elasticity of the bar.

In the table of elasticities actually observed, will be found various numbers between 5' and 73'. To facilitate the comparison of each observed elasticity with the length of the bar on which the trial was made, the following table is annexed, in which the natural sine belonging to each number of minutes has been multiplied by 2.914. he length in inches of the shorter arm of the lever.