pilot balloon

Several thousand observations with various agencies, such as kites and pilot balloons and more especially measurements made with theodolites and nephoscopes, show that there is a definite twist to the right with elevation.

They even took readings high up in the air thanks to a pilot balloon.

My O'D. will, however, serve as a pilot balloon, and if it go up freely we can follow in the same direction.

The pilot balloon, that is so soon out of sight, can offer no guidance—don't forget that!

If I have to perform much longer as a pilot balloon, Tom, I 'm thinking I 'm very likely to explode.