【推荐2】Throughout much of human history, man has been the measure of many, if not all, things. Lengths were divided up into feet and smaller units from the human hand. Other measures were equally characteristic. Mediterranean traders for centuries used the weight of grains of wheat to define (定义)   their units of mass. The Romans used libra, forerunner of the pound, by referring to the weight of a carob (角豆树) seed.

The sizes of similarly named units could also differ. The king's foot, used in France for nearly

1,000 years after its introduction by Charlemagne in around 790 AD, was, at 32.5cm,around a centimeter shorter than the Belgic foot, used in England until 1300.Greek,Egyptian and Babylonian versions of water in a fixed container varied from one another by a few kilos, Nor was there agreement on such things within countries. In France, where there was no unified (统一的) measurement system at the national level, the situation was particularly terrible. The lieue (former measure of distance), for example, varied from just over 3 km in the north to nearly 6 km in the south.

Although John Wilkins, an Englishman, first put forward a decimal system (十进制) of measurement in 1668,it was the French who in 1799 made it law. The Système International d'Unités (SI, or the metric system, as it is better known) developed from it and became the official measurement in all countries except Myanmar, Liberia and the United States. Now the International Bureau of Weights and Measures in Paris is set to give the metric system its biggest shake-up yet.

At a meeting in Versailles, France, on November 16th,2018,the world's measurement bodies are almost certain to approve a decision that will mean four out of the seven base SI units, including the kilogram, will follow the other three, including the metre, in being redefined in terms of the values of physical constants (物理常数).Each of the chosen constants has been measured incredibly precisely, which would mean that from May 20^th 2019 the constants will themselves be fixed at their current values for ever. Any laboratory in the world will then be able to measure, for example, the mass of an object as precisely as the accuracy of their equipment will allow.

【推荐3】Air travel is growing around the globe, and so is its contribution to climate change. The climate cost of flying has drawn more public attention in recent years, even leading to“ flight shame” in some places, especially for local or avoidable flights.

And while a decline in air travel would help with climate change，flight shame can also be added by other ways that make air travel more environmentally friendly. That includes switching to cleaner，renewable fuel , but as a new study indicates , there's also another, less obvious option: flying at lower altitudes.

In a new study, researchers used computers to predict how adjusting aircraft altitudes might reduce the number of contrails(尾迹)，which we see as white lines across the sky containing black carbon particles (颗粒)，thus reducing their warming effect. The study found that aircraft would only need to adjust their altitudes by about 2 ,000 feet, and since some flights have a larger climate effect than others , only a small part of flights would need to make any adjustments.

Of course flying at a lower altitude might increase flying time and it also means burning significantly more fuel, and thus giving off more CO₂, but the benefits of controlling contrails are worth it.

In addition to changing altitudes，better engine technology could also help control contrails, the researchers add, since black carbon particles are produced by incomplete fuel combustion ( 燃烧). With more efficient engines, aircraft could reportedly reduce their contrail by as much as 70%。Combined with slight altitude adjustment for a small part of flights, this could help reduce overall contrail problems by 90%，the study suggests.

This is promising, but more research i still needed, and it may be a while before improvements like these take effect at a significant scale. So, while it's good to know air travel can have a smaller effect on the climate, for now the best way to achieve that is often by simply staying on the ground whenever possible.