Answer.—The best answer to these questions is contained in the reports of the supervising inspectors of the National Board of Health, under whose authority, conferred by an act of Congress, the examination, or inspection, of immigrants entering the United States and traveling therein is conducted. The report of the Supervising Inspector of the Western District, Dr. John H. Rauch, Springfield, Ill., dated Nov. 10, 1882, shows that in this district there were 94,839 immigrants inspected between June 1 and Oct. 31, 1882, of whom 17,195, with or without their consent, were vaccinated, as the case seemed in the judgment of the assistant inspectors to call for:

NumberNumber
Stations.inspected.vaccinated.
Pitts. Ft. W. & Chi. R. R.17,3472,539
Lake S. & M. S. R. R.14,0112,285
Mich. C. R. R.22,3306,145
Grand Trunk R. R.9,3561,875
Balt. & Ohio R. R.10,6882,348
Indianapolis13,7461,635
St. Louis7,361368
Total94,83917,195

Cases of small-pox found on the trains by the inspectors are removed to small-pox hospitals. There were seven such cases in October, 1882, in the Western District.

BRIDGES AND RHYTHMIC VIBRATION.

St. Paul., Minn.

Does music weaken metallic or other bridges? If so, please explain why.

A. M. G.

Answer.—Measured vibrations are more trying to any kind of bridges, and particularly to suspension bridges, than irregular agitation. Music alone would not strain a bridge enough to injure it materially; but a regiment of troops keeping step to music when crossing a suspension bridge would subject it to a very severe strain. Consequently it is customary to stop the music before troops reach the bridge, and let the men break step, and march more or less irregularly. The reason of all this is obvious. The structure will naturally suffer least strain when at rest. When in uniform motion the bridge acquires a momentum equal to its entire suspended weight multiplied by the velocity of the motion. It is manifest that, in the case of a heavy structure, a uniform downward vibration, be it ever so small, would develop a momentum of many tons in the direction of a breaking strain. The same disturbing forces acting irregularly, so as to counteract one another, would be far less trying to the structure.