London's River Thames has twenty-seven bridges．Tower Bridge,the first bridge over the Thames as you travel towards 26 （London） from the sea,is the 27 （most famous） of them all．
The thing that is surprising about 28（Tower Bridge） is that it opens in the middle．It does this 29 （to let ）the big ships through to the Pool of London．If you are 30 （lucky enough） to see the bridge with its two 31 （opening）arms high in the air you will never forget it．
Tower Bridge is less than one hundred years old．It was built in the 32 （1890s）．1850,everyone 33 （agreed） that a bridge 34 （across）the Thames near the Tower London which almost a thousand years old was most necessary．But 35 （the designers）argued about the new bridge for about another thirty years．They took 36（so） long because they had two problems．The new bridge must look like the Tower of London----everyone said so．It 37 （had to）look like a modern bridge．But because of the tall ships it was 38（impossible） to have a modern design．
At last two 39 （clever）designers had the idea of an opening bridge．And they made it look like the Tower．So everyone was 40 （happy）．It was a wonderful success．There was 41 （so）much traffic on the 42（bridge） that the bridge opened at least twelve times a day hundred years 43（ago） ,the River Thames was London's busiest road．
Today big ships 44 （don't） come so far up the Thames．Tower Bridge opens perhaps only twice a week,45 （and）the same wonderful machinery is still 46 （working）.It still has the two 47 （long） opening arms----each 1000 tons----leaving seventy metres for the ships 48 ．And they can still open and close the 49 (bridge）in one-and-a-half minutes．