BLOCKADE 



773 



BLOCK AND TACKLE 



the cold, exhausting wind and blinding snow, 

 human beings and live stock frequently lose 

 their lives in these storms. Numerous instances 

 are recorded of people being lost and frozen to 

 death as a result of trying to find their way 

 between house and barn in a blizzard. 



BLOCKADE, block aid', the patrolling by 

 warships of coasts belonging to an enemy, to 

 prevent the passage of forbidden vessels, in or 

 out. To keep all vessels away from an enemy's 

 country and thus to cripple the foe by shutting 

 out arms, munitions and even food is held 

 to be a legitimate act in warfare. Most na- 

 tions hold that notice must be given of any 

 blockade, that neutral vessels within the block- 

 aded district must be given a reasonable time 

 to leave, that ships attempting to run the 

 blockade (pass through it) may be captured, 

 and that their cargoes are also liable to seizure 

 unless the owners prove themselves not re- 

 sponsible for the attempt. 



Paper Blockade. In 1806 Napoleon declared 

 a blockade against the British Isles (see CON- 

 TINENTAL SYSTEM) and England retaliated by 

 a similar measure against France. Neither 

 country had sufficient ships really to enforce 

 such extensive blockades. They existed only 

 on paper, and the proclamations became merely 

 excuses for capturing ships a mild form of 

 piracy. A "paper blockade" has therefore come 

 to be known as a blockade declared only by 

 publication, or merely a warning to neutral 

 vessels to remain away from a forbidden zone. 

 After the Crimean War the representatives of 

 the powers gathered in Paris declared that 

 "a blockade to be binding must be effective," 

 that is, the blockaders must be able to endanger 

 all ships that may attempt to pass. The Ger- 

 man submarine blockade of the British Isles 

 in the War of the Nations was considered a 

 "paper blockade" by other countries. 



Pacific Blockade. Since 1814 there have 

 been a number of instances when one country 

 has exerted force against another without re- 

 sorting to war, by means of a pacific, or peace- 

 ful, blockade, to secure redress of grievances. 

 Such a measure cannot be enforced against 

 neutral vessels, but only against those of the 

 nation blockaded, and ships cannot be confis- 

 cated, but merely held until the conclusion of 

 the blockade. The English-German-Italian 

 blockade of Chinese waters in 1902 was at 

 first a "pacific" blockade. E.D.F. 



BLOCK AND TACKLE, a mechanical ap- 

 pliance consisting of a combination of pulleys 

 and ropes. It is a machine, for it is designed 



to perform work. Block refers to the casing 

 for the pulleys, tackle, to the ropes. A single 

 block contains one pulley, a double block, two 



,nx to 



iAD 



BLOCK AND TACKLE 



Fig. 1. (a) No mechanical advantage; (6) 

 mechanical advantage of two; (c) mechanical 

 advantage of four. 



pulleys, and so on. Each block usually has a 

 hook with which to fasten it to its support or 

 to the object to be moved. 



In the article PULLEY it is shown that a 

 simple movable pulley, such as is contained in 

 a single block and tackle, has a mechanical 

 advantage .of two; that is, with its help a force 

 will move practically two times the weight that 



BLOCK AND TACKLE 

 Fig. 2. The endless chain. 



it can move without it. The mechanical ad- 

 vantage of a double movable block is four, 

 for, as shown in Fig. 1 c, there are four ropes, 

 each of which bears one-fourth the weight; 

 therefore any pull on the end in excess of one- 

 fourth the weight will lift the object. Simi- 

 larly, the advantage of a triple movable block 

 is six. 



Endless Chain. The apparatus shown in 

 Fig. 2 is technically known as the differential 



