Smoky chimneys are usually caused either by the presence of other buildings obstructing the wind and giving rise to irregular currents of air, or by improper construction of the fireplace and adjacent parts of the chimney.

The first may generally be cured by fixing a chimney-pot of a particular construction, or a revolving cowl, on the chimney top, in order to prevent the wind blowing down; in the second case the narrowing of the chimney throat will generally create a better draft.

The longer a chimney is, the more perfect is its draft, provided the fire is great enough to heat the column of air in it, because the tendency of the smoke to draw upwards is in proportion to the difference of weight between the heated air in a chimney and an equal column of external air.

The first we hear of chimneys, for the escape of the smoke from a fire or furnace, is in the middle ages.

Floating Dry Dock "Dewey".

This dry dock, which is capable of floating the largest battleship, was towed from Sparrow's Point, Maryland, to Olangapo, Philippine Islands, a voyage of 13,000 miles. In operation, the dock is sunk by admitting water into its tanks until the ship can be floated m. The water is then pumped out and the dock with the ship inside rises to the proper level as shown.

U. S. Battleship "Mississippi" in Dry Dock at League Island.