Monthly Archives: August 2014

2013-2014 Review launched on 30th August 2007. In 2012 I published a Five Year Review, and last year I reviewed 2012-2013, and I thought I’d follow up this year with a review of 2013-2014.


September, like many months this past year, was a quiet one for posts. I looked at where all the elements came from, the differences in the order of subjects, objects and verbs in sentences in various languages, and finally worked out the difference between “tons” and “tonnes”.

Full September archive


I was quite pleased with a post about tritium illumination and telomeres and aging, but my favourite post from October was one about the different types of average. I also wrote about an actual method of turning lead (or something else) into gold.

Full October archive


I liked all my posts from November. The post about different sorting algorithms was probably a bit too long, and could maybe have been a bit clearer, but the ones on the different gases used when breathing underwater and on RAID array types were just right.

Full November archive


The post about the EURion Constellation and counterfeit money was easily my favourite from December, and one of my favourites from the whole year. I also liked Earth Sandwich and the post about inequality and the Gini Coefficient.

Full December archive


There was nothing really outstanding from the month of January, but I did enjoy researching Unconventional Nuclear Weapons.

Full January archive


February was very quiet, with only two posts: one on converting from miles to kilometres by using the Fibonacci sequence which was more popular than I’d expected it to be, and one on decibel weighting and the difference between white and grey noise.

Full February archive


March was not a good month for


April was much better than March, in that I actually made some posts. joined Twitter, and I wrote another personal favourite post discussing what “Five Sigma” data is. I also corrected a very common misconception, writing about the difference between real and apparent weightlessness.

Full April archive


May was another busy month, with a definite typographic theme towards the end. My personal favourite was Why Tokyo Looks Different From Space, but I was also pleased with Ranking Ratings and UPC Barcodes.

Full May archive


An amazing coincidence in June, when an issue I wrote about in a post (Cousins or Siblings?) came up in an A Level biology final exam question a few days later. I also wrote about where space starts, and the shape of rocket engines and the creation of “shock diamonds”.

Full June archive


July wasn’t a brilliant month, because I was very busy at work. I’m still not quite sure I explained why we can’t get all the way to absolute zero quite well enough, but it’s a post I quite like.

Full July archive


I found The Composition of Earth’s Atmosphere with Elevation very interesting, but I have a feeling I might be the only one. I was also very interested by hydraulic fuses, and I finally worked out the difference between an assault rifle and a carbine. I also explained why it feels hotter when the air is more humid.

Full August archive

Here’s to 2014-15. I hope you’ll keep reading.

Propellers and Impellers

Propellers and impellers both provide thrust, but do it in different ways.

A propeller is a fan which propels a fluid by pushing against it: it converts rotational motion into linear motion. An impeller is a rotor that produces a sucking force, and is part of a pump. A propeller is always “open” and an impeller is always “closed” (as it has to draw fluid into something).


L-R: Impeller and propeller


Human beings judge how warm they are by how quickly heat leaves the body (we have specialised cells called thermoreceptors that do this job), and maintain temperature by controlling sweating. As sweat evaporates from the skin, it takes heat away from the body and therefore the rate of evaporation affects the rate of heat loss.

When the air is very humid, it is more difficult for evaporation to take place, and therefore humidity affects people’s perception of the temperature: when the air is very humid it feels warmer than it really is. Humidex is an index used by Canadian meteorologists to calculate this apparent temperature.

The dew point is the temperature at which water condenses out of the air and is linked to the relative humidity. If the relative humidity is 100% then the dew point is equal to the current temperature; when the current temperature is below the dew point then water will leave the air (the dew point cannot be higher than the current temperature).


As you can see, as the dew point increases the perceived temperature increases. When the air temperature is 20ºC and the dew point is also 20ºC  then the perceived temperature will be 27.6ºC.

The hottest and most humid parts of the world are the coastal areas around the Arabian Peninsula. On July 8th 2003 in Dharan, Saudi Arabia the air temperature was 42.2ºC with a dew point of 35ºC; this yields a humidex of an almost-unbearable 68.9ºC. If we combined the hottest recorded temperature of 56.7ºC in Death Valley, California with this Saudi Arabian dew point record that would yield a humidex of 83.4ºC, which would have killed people: luckily the Death Valley area is a desert valley and the dew point is usually quite low.

Lift Test Towers

How do you test a lift? Build a big tower, of course.


The world’s tallest lift test tower is Hitachi’s G1 Tower in Ibaraki, Japan. It is 218.5 metres tall (which would make it the fifth tallest building in the UK) and has room for seven different elevator shafts operating at speeds over 1000 feet per minute or 5.5 metres per second.

The world’s fifth largest lift test tower is the National Lift Tower in Northampton, at 127.5 metres tall.


Hydraulic Fuses

A hydraulic fuse acts to stop the excessive flow of a hydraulic fluid, in the same way that an electrical fuse acts to stop the excessive flow of electrical current. They are commonly found in “mission critical” hydraulic systems, such as those that operate the flight control surfaces (ailerons, flaps, rudder) on aeroplanes.

There are two types of hydraulic fuse. The first operates as a pressure relief valve, and vents fluid in case of a build-up of pressure. The second operates to prevent the loss of hydraulic fluid, for example if a fluid line is severed, and operates as a check valve – allowing fluid to flow only in one direction.



In the example of the second type of fuse shown above, excessive flow through the inlet will push the piston between the two metering plate housings and into the outlet, preventing fluid from passing through the fuse. The spring prevents the fuse from operating too early, pulling the piston to the left against the pressure to the right.

The fuse plugs used in hydroelectric dams can be viewed as a type of hydraulic fuse. They are usually constructed from across dam spillways, preventing water from exiting the reservoir along the spillway. In the event that the water level rises too high, the fuse plug is washed away by the increased water pressure.


The fuse plug at the Warragamba Dam in Australia.

The fuse plug is the large dark grey structure in the bottom-centre of the map. In the event that the level of water in Lake Burragorang on the left rises too high, the excess water will wash away the fuse plug and run down the light brown spillway towards the top-right.