built, consisting of arch spans and piers, or steel bridges resting on concrete piers, some spans of the latter being two hundred and forty feet. This was the most difficult part of the work. The water is from ten to thirty feet deep, and the bottom is coralline rock. There are twenty-eight of these arch viaducts, aggregating ten and eight tenths miles in length, and eight steel bridges, aggregating six and one tenth miles in length.

The longest viaduct is between Knight's Key and Little Duck Key, seven miles, and is called the Knight's Key Viaduct. In many places the embankment for the roadway is eight or nine feet in height, the road-bed being ballasted with coralline limestone, of which these islands are composed. This makes a very strong, safe road. In many places where the water is deep enough

to float an ocean steamship of large size, and where the locality is exposed directly to the gales from the Atlantic, much of the work has been performed with floats, on which the concrete was mixed and from which it was placed in position by means of powerful derricks. In the shallower waters molds for the foundation of the viaduct were formed by driving piling which held in place a water-tight framework, which, when the water was pumped out, was filled with concrete.

This, the only railway of the kind in the world, is now in actual operation, and reflects great credit on modern enterprise and skill. Both land and sea are laid under tribute; and these islands, which have been likened to lazy lizards sleeping through uncounted centuries, now teem with life and thrill with the rush of commerce.

main at the same height, the horizontal bars remain in the same relation to the eyes and to the object on the other side, and make a strong impression on the retina. As they are darker than the background, they produce a sharp afterimage when the eyes are turned away. There seem to be several horizontal lines in the afterimage because you probably look for a while at the upper part of that object on the other side, then for a while at the middle part, then for a while at the lower part. With every raising or lowering of the eyes, the image of the horizontal bars falls on another spot in the retina, and leaves there the condition for another after-image, producing in this way a series of parallel horizontal lines.-H. M.

NOTE: A scientific friend says he has observed that when an automobile passes under a light at night, the wheels seem to him to run backward.

WHEN SPRING "PEEPERS" ARE HEARD PURCHASE, N. Y. DEAR ST. NICHOLAS: Can you tell me why peepers do not peep in the morning? If you can, I wish you would tell me.

Your interested reader,

BENJ. COLLINS, JR. (age 10). Frog "peepers" are nocturnal animals, and seldom active except during the late afternoon or at night. I have heard them calling during the day, but they usually begin to evince signs of in

terest in things as the day comes to a close, and continue the calls during the night.-RAYMOND L. DITMARS.

WHY BRUISES BECOME "BLACK AND BLUE" NEW YORK MILLS, N. Y. DEAR ST. NICHOLAS: I have often wondered what makes black-and-blue marks on your skin when you are bruised. I take physiology and am very much interested in it, but my book fails to answer this question. I should be very much obliged if you would tell the reason.

Yours as ever,

LOIS W. KELLOGG (age 13).

The color of blood is due chiefly to iron in the little blood-cells. When the iron is kept in these little blood-cells, which are living and traveling around in the blood-vessels, the color is red. Hit the skin hard enough to break some of the little blood-vessels beneath the surface, and the little red cells escape from the injured blood-vessels, wander about for a while in the tissues, and die. When they die, the iron that made them red before, then changes to black-and-blue coloring. After a while, this iron is taken up by the glands called the lymphatics, and made over again into nice red cells. The iron is taken up very much more quickly by the lymphatics if the black-andblue spot is rubbed and massaged. -DR. ROBERT T. MORRIS.

FRAGRANT FIREWORKS

WORCESTER, MASS. DEAR ST. NICHOLAS: Orange-peel fireworks are great fun. Papa and my brother Roland squeeze the rinds and I hold the match. Sometimes when the orange peel is

FIREWORKS FROM FLOWERS

THE gas-plant (Dictamnus), which has fragrant leaves and bears curious flowers, "gives off during hot weather a fragrant, volatile oil, which ignites when a match is applied to it."

Mr. Nathan R. Graves, Rochester, New York, sends the accompanying illustration of the bloom. He writes:

"I have found that the flash, when a lighted match is held near to the bloom, is more certain on a sultry evening after a very warm day. Then one seldom fails to get quite spectacular results."

The gas-plant is attractive and of value aside from its peculiar inflammable gas. The seeds should be sown in the autumn in a plant nursery bed where they are to remain for two years.

THE BLOOM OF THE GAS-PLANT, OR BURNING BUSH.' (Dictamnus fraxinella.)

In dry, sultry weather the flowers sometimes give out a vapor which is inflammable.

They can then be transplanted to rich, heavy soil. They bloom in the months of June and July.

[The fireworks with orange peel and flowers have in themselves no danger, but, because matches are so common, one should never grow careless in the use of them, even to light a lamp.-E. F. B.]