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LEFT STREET RAILWAYS 119 STREPSIPTERA in this country. It was their inventions and experiments which were to make street railways as universal as they are in American cities to-day. The first electrically driven tramway was exhib- ited at the Chicago Industrial Exposi- tion, in 1882, but it was not till six years later, in 1888, that the first elec- tric railway system was put into opera- tion, in Richmond, Va., where a line twelve miles in length was laid by Frank J. Sprague, the "father of Amer- ican street railways." The enterprise in Richmond proved both mechanically and financially successful, and hence- forward electricity became more and more the universal motive power for American street railways. Ninety-nine and one-half per cent, of street railways in this country are operated by elec- tricity. Another side development of street railways in this country, such as was the cable-car, has been the elevated rail- way. As far back as 1860 the difficulty of operating a tram service along the congested thoroughfares of New York City was brought up for solution. Two alternatives only presented themselves: the tracks must either be laid along an underground tunnel, or raised above the streets on a superstructure. Both schemes were discussed, but the subway proposition seemed too expensive to justify itself. In 1866 construction was begun on the first elevated railway in New York City, running from the Bat- tery to Thirtieth Street, along Green- wich Street and Ninth Avenue. For a while the cable system was used as mo- tive power, but very soon after steam locomotives were installed. Steam re- mained the motive power until 1902. The enterprise proved a commercial failure, but elevated railways were ob- Tiously a solution to the problem of con- gested streets in big cities. Boston, Chi- cago and a number of other large cities followed the example of New York and reared elevated superstructures along their streets. In 1920 there remained only nine local street railway systems in American cities which operated their lines by means of animal power. There was, in the beginning of the year, over 50,000 miles of street railway tracks, carrying on an average of 150,000 passengers ])ei' mile each year. Annually the number of passengers carried amounted to fif- teen billions. During recent years there has been a tendency in the larger cities away from private to municipal ownership in the operation of street railways. In 1911 Seattle took over 203 miles of street railway track, at a cost of $15,000,000. In 1920 San Francisco owned 64 miles of track and 195 passenger cars, as com- pared to 286 miles of track and 700 cars operated under private management. Whether electric street railways, op- erated along permanently laid tracks, will develop in the future as they have in the past few years, seems to depend in no small measure on the extent of the National sources of supply of gaso- line. In many cities, and especially in the suburban districts, gasoline driven motors buses are competing with the street railways so successfully that in many instances the electric lines have been abandoned. In New York City sev- eral old street railways have been abandoned in the downtown section of the city and bus lines established to take their places. STREHLENAU, NICOLAUS NIEM- BSCH VON. a German poet, born in 1802. STRENGTH OF MATERIALS, in mechanics, an important part of the theory of structures, and includes the consideration of all questions relating to the straining of solids. A solid is said to suffer strain when it is in any way altered in volume or figure, and to eff"ect such an alteration external force must be applied. If the strain is not carried so far as to cause disintegration of the solid — that is, if no fracture results — the force or resultant of the forces which produce the strain must be bal- anced by an internal force conditioned by the molecular structure of the solid. This internal force called into existence by the strain is the one aspect of the stress, of which the original force producing the strain is the other. To every strain, then, there corresponds a stress, which is regarded as the resultant of the stresses which act between the elemen- tary portions into which the solid may be supposed to be divided. When the strain is proportional to the stress — in other words, when Hooke's law holds — the body strained is said to be perfectly elastic ; and the elasticity of every solid, even for such cases as moistened clay, is sensibly perfect when the strain does not exceed a certain limit. For every solid, however, there are limits of strain which, if exceeded, give to the body a set or permanent alteration of volume or figure. Hodgkinson has proved that these limits depend on the duration of the strain, so that in all probability any strain if sufficiently long-continued would give a set to any body. STREPSIPTERA, a small and very peculiar and anomalous order of insects. The females are wingless, and live as