Most of us know balloons from festivals and fun fairs, where they are colorful and amusing, but in the 19th century, before airplanes were invented, balloons were used in wartimes to spy on the enemy position.
我们大多数人都知道，人们在节日或游乐园里会放飞丰富多彩的气球。但在飞机发明之前的19世纪，气球在战时被用来对敌人位置进行侦探。

One or two men would go up with a telescope in a balloon tied to the ground with a rope.
一到两个人会带着望远镜进入气球，升到空中，而气球则绑在地面的绳子上。
However, because the enemy might fire a cannon at them, this was dangerous.
然而，因为敌人可能用大炮向他们开火，因此这相当危险。
So before men would go up in a balloon, they would first float (or send up) a trial balloon without any men in it to test the situation.
所以，在侦查人员进入气球之前，他们会首先放起（或发射）一个试探性气球（里面没有任何人）来试探一下情况。
If the enemy tried to shoot that trial balloon, the men would not go up.
如果敌人试图攻击这个试探性气球，侦查人员就不会再坐上去了。
From this origin, today we use this idiom in any situation where we want to measure public reaction or opinion before committing a lot of money and effort to some project.
正是来源于此，在我们给某个项目投入金钱或精力之前对公众的反应或意见进行试探的情况下，都可以使用这个成语。