Page:Science vol. 5.djvu/371

 LH, 1885.]

��) Th&t the elastic yielding of the ballast under the ~ pMSJDg loads, and the slight rocking of the ties, ab- sorb or resist the creeping (orce, would appear from the fact [hat the tendency to creep Is most pro- nounced where the supports uoder the rails are held rigidtj, as in bridgea. On the Uarrisburg bridge, over the Suiquehanna, the Pennsylvania company encounlered Ibis dlftlculty, but arrested the raove- ment by spikes through the angle-splices at joints. On the St. Louis arched bridge, and Its east approach, there is found a most remarkable example of creep- ing rails. Prof. J. B. Johnson, in a paper read before the Eniilneers' club of St. Louis,' discusses this case at lenj^th, and offers an eiplanalion.

The bridge proper is 1,000 feet long; the east ap- proach, a series of short girders on iron columns, is •i.SOJ feet long, with a graile rising towards the bridge of eighty feet per mile; both are double -tracked. A» it was thought by those in cliarge o( the bridge that fastenings at frequeut Intervals, to resist the movement, would bring too great n strain upon the structure, the attempt was made to restrain the rails by holding them firmly at isolated points some dis- tance apart, with the result that spikes, bolls, and splice-liars were sheared off or torn apart. After the (allure of att«mpts to arrest the creeping, the track was cut at the two abutments and at the east end of the east approach. The time of eight men {five by day, and three by night) is slated to be largely nccupifd iu changing rails at these poinis. Where the openings are enlarging, sliort pieces of rail are taken out, and longer ones put in their place; where the openings are closinj; up. ihe process la reversed. Each operatioik Is performed many times a day, and a careful record Is kept, from which the following facts wura obtained: the north track, when carry- ing an annual westward traffic of about 5,283,000 tons, moved west on the approach and up-grade 401 feet in a year, and on the bndge moved i!«4 feet; the south track, under an eastward Irafiic of 4,607,000 tons, crept east 414 feet on the approach, and 240 feet on the bridge. In the same time. The niovement each way on the bridge was proportional to the ton- nage; and the difference on the appruach was doubt- less due to the grade, as the changes of temperature would produce a slipping down hill, as previously

Professor Johnson cites some explanations of this case that have been given: viz., the stopping of trains on the bridge; the deflection of the bridge itself by the weight of the train ; the distortion of the arcli, aa a train enters a span, by its curve becoming leu convex on the loaded portion, and more convex on the unloaded side, with a reversal of the distor- tion u the train passes over and oft the span, the arch thus slipping under the rails; and, finally, the elastic rolling-oui and recovery of the rails under successive wheels, as we may Imagine a strip of rub- ber to move aa a roller is passed over it. He does not think, however, that these causes are sulQcient to account for so great a movement, and, in eiplun-

> Jorarn^ of Iba AiwkIiIIod of CDglnevtinn Kclvllea. Muvem.

��ing bis theory, '>ffer' a preliminary illilstralion. Sup- pose a Hpnu of a bridge to have aupports exactly alike, such as sliding surfaces, at the ends of the bottom chord, and a train to enter upon It. The bottom chord is stretched by the action of the load, and, as the end where the engine enters is held fast by the added weight, the other end must slip on its support in the direction of the train movement. As tbecars pass off at this latter end, uid hold it fast, the lower chord shortens, and recovers itself at the first bearing by slipping towards the train. Thus the bridge creeps in the direction of the moving train. If the points of support were under the upper chord, the direction of this creeping would be reversed. When rollers are placed under one end, and the other is anchored fast, the slip and recovery take place on the rollers, and no creeping results.

He notes that between the trucks of every car the rail springs up from the support an appreciable dis- tance, by reason of the elasticity of Its bearings, and that, when pressed down by the passage of the rear truck, any marked point on It has advanced a small distance. A wave-motion of the rail may be per- ceived In advance of every wheel, and an Increment of forwanl movement every time a wheel passes. The more cars, the more movement for any train. The rail moves across the bridge by reason of the extension undei- flexure of the flange on which It rests. In proof of his position, he showed, by a model over which a loaded wheel was rolled, that a rail supported by the bottom flange will creep forwards, and tbat tlie same rail, when supported by Its head, will creep backwards; and hence he ai^ues that some point of support between the heasition of the patriarchal tlieory of the origin of society, which had long been held by a class of writers in Europe. In his introduction he says, —

" This evidence establishes that view of the race which is known as the patriarchal theory. This theory ia based on the scriptural history of the Hebrew patnarchs. All known societies were originally organized on this model. The eldest male parent is absolutely supreme in his household. His dominion extends to life and death, and is as unquatified over his children

�� �