Babbage

HOW to make money while offering your wares free of charge? On the internet, the answer typically involves selling advertisers access to users or their data. This tactic will not yet work on App.net, a fledgling fee-based microblogging service, which so far lacks the number of users for ads in any app to produce much income. Yet Paul Haddad managed to pull it off.

Mr Haddad is the co-founder of Tapbots, the maker of Tweetbot, a popular Twitter app for Apple devices. A mobile version of Tweetbot costs $2.99, three times the lowest and common price for much software on Apple's iTunes online store. The App.

TWO years ago IBM attracted a lot of admiring publicity when its “Watson” program beat two human champions at "Jeopardy!", an American general-knowledge quiz. It was a remarkable performance. Computers have long excelled at games like chess: in 1997 Deep Blue, another of the computer giant's creations, famously beat the reigning world champion Garry Kasparov. But "Jeopardy!" relies on the ability to correlate a vast store of general knowledge with often-punny, indirect clues. Making things hardest still, the clues themselves are, famously, phrased as answers, to which contestants must supply an appropriate question.

ALTHOUGH undeniably graceful, gliding has until now been suitable only for pleasure flights. But this is changing, as researchers exploit wind power to enhance the capabilities of unmanned aircraft, especially small drones. Soon, these gliders will be able to stay aloft for weeks. They will thus be able to act as communication relays, keep a persistent eye on the ground below and even track marine animals thousands of kilometres across the ocean.

One such glider, the hand-launched Tactical Long Endurance Unmanned Aerial System (TALEUAS) is being developed at the Unites States’ Naval Postgraduate School in Monterey, California.

SUICIDE is a part of life. Whenever any of the 100 trillion or so cells that make up the human body malfunction, which happens all the time even in healthy tissue, they are programmed to provoke their own death. The mechanism hinges on a protein called TRAIL, which is produced by the damaged cell and binds to receptors on its surface, causing inflammation. That is a signal for the immune system to sweep in and, through a process called apoptosis, break down the damaged cell and recycle its parts to feed healthy ones. If this self-destruct is subverted, however, the result is a tumour.

THE Graphics Interchange Format (GIF) is the brainchild of Steve Wilhite. He invented it in 1987 while working at CompuServe, a time-sharing system that originated before the internet, and was open to anyone with a dial-up modem. In those days, when transfer rates topped out at 2,400 bits per second, less than a thousandth of what modern DSL or cable connections can manage, minimising the number of bits dispatched was vitally important. GIF reduced the colour palette and compressed data to achieve just such savings. GIF also dominated JPEG, a rival format best suited for photographs, in the web's early days because of its compactness. That much everyone agrees on.

AS A rule of thumb, the average number of miles most American motorists get per gallon is a good mile or two less than the “combined” figure printed on the window stickers fixed to new cars in dealers’ showrooms. It used to be a lot worse. But from the 2008 model year onwards, the Environmental Protection Agency (EPA)—the body that devises the fuel-economy tests and checks the data that vehicle manufacturers provide for the government’s Corporate Average Fuel Economy (CAFE) programme—revised its testing procedure to match peoples' driving habits much better.

At its laboratory in Ann Arbor, Michigan, the EPA does not check all the test results of every make and model for accuracy.

OUR correspondents discuss cyber-security, turmoil at the Royal Institution and Twitter's new video-sharing app, Vine