it about a sphere that makes it round?

但什么才是"圆"呢

If you're talking about how effective a shape is at enclosing a large volume, you mean "sphericity".

我们用"球性"这个词

And a sphere is the shape that contains the most volume with the least surface area, that

来描述一个形状围住容量的有效性

is, a sphere is the most "spherical" shape.

显然球形能利用更少的面积围住更多体积

But roundness has more to do with rolling than with volume, right?

也就是说球是最有"球性"的形状

A round object can roll smoothly - like a wheel or ball bearing.

但圆常常被用做移动而不是装物体

And the main feature that ball bearings need in order to roll smoothly is to be the same

圆形的物体可以平滑的滚动 比如轮子和滚珠

width from bottom to top/one side to the other.

而它们最大的特点就是在滚动过程中 顶部到底部的距离不变

It turns out there are plenty of non-circular 'shapes of constant width' that are excellent

事实上许多不是圆形的形状也可以用作轴承

bearings as well.

比如 勒洛三角形

The Reuleaux rotor, for example, is made up of segments from three circles intersecting

它是由三个交叉圆的圆心连接而成

at their centers - so every point on a side is the same distance from the opposite corner,

形上任何一点到对边的距离都相等

and the rotor rolls around wonderfully.

所以它也能平滑地滚动

In fact, the Reuleaux rotor can even turn smoothly in a square hole!

事实上 勒洛三角形甚至能在正方形的洞中平滑地滚动

But don't try to use a Reuleaux rotor for the wheel on your car - since they're not

但你别指望它代替车轮

a constant distance from the axle, those points will make for a bumpy ride!

因为它能颠的你够呛

In fact, the points on a Reuleaux rotor are just that - pointy.

勒洛三角形的顶点都很尖

Doesn't that go against the idea of roundness?

这是否意味着它同时具有磨圆度呢

Geology has the answer: stones with sharp or rough edges that are worn away become "rounded."

在地质学中 这又是另一码事

So we might say a Reuleaux rotor is round but not rounded.

磨圆度是指石头被磨圆的程度

On the other hand, certain British coins are round AND rounded, though they're still not

所以我们说勒洛三角形有"圆性"但没有磨圆度

circles: the 20 and 50-p coins are shapes of constant width, which means they look un-circularly

而一种英国的硬币既有"圆性"也有磨圆度

cool but don't get stuck in coin machines!

尽管它们也不是圆形

And isn't if funny that "rouleau" means "roll" in French and Franz Reuleaux invented a rolling

这种20和50的便士有7个等边

rotor?

也就是说它们不会卡在投币机的狭缝里