348 



THE CENTURY BOOK OF FACTS. 



forced to be abandoned. Sawmills were erected 

 near London about 1770, and thenceforward 

 became general. 



Screw Propeller, The. In 1802 Dr. 

 Shorter, an English mechanician, produced 

 motion by the agency of a screw ; but his dis- 

 covery was of no value at the time, as the 

 steam engine had not then been applied to 

 navigation. In 1832, Mr. B. Woodcroft pat- 

 ented a screw propeller with an increasing 

 pitch ; and four years later Mr. F. P. Smith 

 patented a screw making two whole turns, 

 which he reduced in 1839 to one whole turn. 

 In 1837 he and Captain Ericsson brought the 

 matter practically forward on the Thames, 

 where a small screw steamer, forty-five feet 

 long, eight feet broad, and of twenty-seven 

 inches draught, towed a . vessel of six hundred 

 and thirty tons against the tide at four and 

 one half knots an hour. This experiment was 

 followed by a number of others, some un- 

 dertaken under the direction of the British 

 Admiralty, which clearly established the 

 practicability of the screw, and its advantages 

 for ships of war became incontestable. From 

 the entire submergence of the propeller, and 

 the consequent lowness of its engines in the 

 ship, the chances of injury from an enemy's 

 shot were reduced almost to nothing. The 

 screw propeller is of the same construction as 

 the common screw, but with the narrow thread 

 exaggerated into a broad, thin plate, and the 

 cylinder diminished to a mere spindle. If a 

 screw of this form were turned round in an 

 unyielding substance, as wood, it would for 

 each turn advance as much as the center of 

 the blade (or thread) had moved along the 

 spindle in forming the screw, i. e., the dis- 

 tance. If, on the other hand, the screw itself 

 were prevented from moving longitudinally, 

 and the piece of wood not fixed, the latter 

 would be compelled to advance along the 

 screw the same distance. When the screw is 

 fixed beneath a ship and made to revolve in 

 the water, the case lies between the two just 

 supposed the screw moves forward, and with 

 it the ship, and the water in which it has been 

 working moves backward. The backward 

 motion should only be small proportionately, 

 and the ratio between it and the sum of the 

 backward motion of the water and the forward 

 motion of the ship is called the slip. Screws 

 have been formed with two, three, four, and 

 six blades, or arms ; but the form most com- 

 monly used is two blades for ships of war, and 

 three or four blades in the merchant service. 



Ship Building. This art is attributed 

 to the Egyptians, as the first inventors ; the 

 first ship (probably galley) being brought from 

 Egypt to Greece by Danaus, in 1485 B. C. 



The first double-decked ship was built by the 

 Tyrians, 786 B. C. The first double-decked 

 one built in England was by order of Henry 

 VII., 1509. It was called the Great Harry 

 and cost 14,000 pounds. Portholes and other 

 improvements were invented by Descharges, a 

 French builder at Brest, in the reign of Louis 

 XII., about 1500. Ship building was first 

 treated as a science by Hoste, 1696. Iron is 

 now greatly used in ship building. For beau- 

 tiful models and fast sailing, the shipping of 

 the United States (especially the packet ships 

 and steamers sailing from New York) is not 

 surpassed, and probably not equaled, by that 

 of any other nation in the world. 



Signals, Wind and Weather. A red 

 flag with a black center indicates that a storm 

 of marked violence is expected. A yellow flag 

 with a white center indicates that the winds 

 expected will not be of extreme severity. A 

 red pennant indicates easterly winds that is, 

 from northeast to south, inclusive, and that, 

 generally, the storm center is approaching. If 

 shown above the red flag, winds from the 

 northeast are more probable ; if below, winds 

 from the southeast may be expected. A white 

 pennant indicates westerly winds that is, 

 from north to southwest, inclusive, and that, 

 generally, the storm center has passed. If 

 shown above the red flag, winds from north- 

 west will probably prevail; if belo^, winds 

 from southwest. A white flag indicates fair 

 weather. A blue flag indicates rain or snow. 

 A black triangular flag refers to temperature ; 

 when placed above the white or blue flag it 

 indicates warmer weather ; and when placed 

 below them, colder weather. A white flag 

 with black square in center indicates the ap- 

 proach of a sudden and decided fall in tempera- 

 ture, and is usually ordered at least twenty-four 

 hours in advance of a cold wave. When dis- 

 played on poles, the signals are arranged to 

 read downward ; when displayed from horizon- 

 tal supports, a small streamer is attached to 

 indicate the point from which the signals are 

 to be read. 



Silkworm. It is the general belief that 

 the great importance of the silkworm was first 

 discovered by Se-ling, the wife of the Chinese 

 Emperior Hoangti, who reigned about 2637 

 B. C., and that she also invented and taught 

 the art of silk-spinning and weaving. The 

 worms are exceedingly tender, and liable to 

 perish from the slightest changes of tempera- 

 ture and dampness. They feed upon the leaves 

 of various trees and bushes, but experiments 

 go to show that the best silk is produced when 

 the worm is fed upon mulberry leaves. The 

 great centers of this industry are China, Japan, 

 India, and Southern Europe, and they have 



