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thus formed would have the same vertical rigidity as the existing compression arches, and it is obvious that the lateral bracing which is necessary for the tubes of the erect arch could be dispensed with for the links of the suspended arch. The question of anchorages is outside of the comparison.

The popular misconception as to suspension bridges is due to the many insufficiently stiffened structures of this kind. No other bridge system can be built so imperfectly stiffened, and yet be safe, as the suspension bridge. An erect arch bridge built in the same manner would fall of its own weight.

Another popular and fashionable conception, but a misconception all the same, is as to the merits of the cantilever bridge. Theoretically and practically, the cantilever of all bridge systems has the greatest deflections and oscillations under passing loads, all other things being equal, and therefore is the least rigid system. It has, however, its good uses otherwise.

Gustav Lindenthal.
Pittsburg, Pa., February 2, 1890.


Editor Popular Science Monthly:

Dear Sir: In your issue of January, 1890, page 430, under "Notes," it is said, "One hundred and fifty-five barrels of salt were manufactured in Kansas in 1888, and it is estimated that the output in 1889 will be not less than three times as large."

From the annual report of the Secretary of the State Board of Agriculture for 1888, it is learned that seven salt-works reporting produced 122,420 barrels. Of the seven, three reported to December 31st, and four to November 30th. One of the seven reporting began March 15th, three in October, and two in November.

From the same source for 1889, 547,224 barrels of salt were manufactured and 19,056 tons of salt not put in barrels. Seventeen companies reported in the latter year.

I have bought a copy of "The Popular Science Monthly" since its first publication, and I was loath to pass such an error unchallenged. Success to you and yours.

J. G. Wood.

Topeka, Kansas, February 11, 1890.

[The number intended was 1 55,000 barrels. The dropping out of the thousands in transcribing the item escaped notice. On the basis of that number, the output of Kansas salt in 1889 would be, according to Mr. Hay's estimate, not less than 465,000 barrels. We thank our correspondent for giving us the opportunity of correcting the error.—Editor.]



AMONG the regularly constituted sciences that claim the attention of the world to-day, it can scarcely be said that political economy has an undisputed place. Fourteen years ago, in an article on the centenary of the "Wealth of Nations" (which fell in the same year as the centenary of our Declaration of Independence), the late Prof. Jevons acknowledged that there was then far less agreement among teachers of political economy, in regard to the fundamentals of their subject, than there had been fifty years earlier. He acknowledged, also, how little interest was taken in lectures on political economy at the universities, and how little weight was attached by practical men to propositions or principles put forth as the result of studies in that field. Row does the matter stand now that fourteen years more have flown? Has the credit of the economists of the generation that has passed away—the Mills, the McCullochs, the Seniors, the Says—been in any degree rehabilitated? Scarcely. As time goes on, it becomes increasingly clear that the whole work of these writers was carried on too much in the region of abstractions, and was too little vivified by direct contact with facts. Bacon long ago remarked on the error of those who supposed in nature a greater simplicity than really exists; and this error was abundantly exemplified by the classical or "orthodox" economists. It was to certain minds, no doubt, a fascinating pursuit to seize upon two or three general principles, and by their help to interpret and methodize all the complex phenomena of economic production, distribution, and exchange; but the process was hazardous in the