sounding-balloon

from The Century Dictionary.

• noun A small balloon bearing meteorological apparatus, but without an aeronaut, sent rapidly up to the highest possible elevation in order to ascertain the atmospheric conditions. The balloon is carried off by the wind and comes to the ground at a long distance, and the record is found and brought back. Two forms of sounding-balloons are in use. One, the older, usually of silk or paper, falls when sufficient hydrogen gas escapes through the open appendix; the other, of rubber, sealed, expands as it rises and eventually bursts, the apparatus falling slowly, supported by a parachute. Originally and frequently called ballon-sonde.