This logically should have gone into the previous post about archives, but I got carried away working out what that air mail poster was about! But I had intended to mention two online archives of British newsreels: British Pathe and Movietone (slogan: "It speaks for itself"). These are great. You can search the descriptions for key words - Hendon, say, or "air raid" (or even something not aviation-related, if you are so inclined!) - and turn up all sorts of gems, like a 1923 reel showing off 'London's air defences',
The best part is that you can view (and often hear) the newsreels for free! If you wanted to use stills or clips in a documentary or publication, you'd have to pay. However, the online previews should be fine for most research purposes (and you can even save the British Pathe ones onto your hard drive). The search engines and the video playback can be cranky sometimes, but if you start again it will probably work better.
There's a good overview of the history of the British newsreel at the British Universities Film & Video Council, including summaries of the different series that were made, what has survived and where they can be found. There are still several major newsreel titles that don't appear to have been digitised yet (eg Gaumont, Paramount); hopefully that's only a matter of time. Newsreels were an important news medium until well after the Second World War. They had a weekly audience of millions and had an immediacy that radio and newspapers could not match (on the flipside, though, they lacked the timeliness of the former and most importantly the depth of the latter). These digitised archives make it that much easier for the historian to understand just what was being presented to the public in the many thousands of newsreels that were produced up to 1979.
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