You may be referring to the use of smudge pots which is a method of using oil burning heaters in orchards to prevent freezing of the crops.
eBay or your local classified advertisements are the best bet, since I don't think there are any US manufacturers anymore. Smudge pots are such an environmental nightmare, and there are much better and more cost-effective methods available. Simply starting up water sprinklers over the trees when there is frost danger will take care of them as well as smudge pots would. Orchard fans are another possibility.
No. If it is written with pencil it can be erased and re-written without any smudge and nobody can detect it. hence it is not legal
They do not work by heating the air. They work by producing a heavy smoke layer over the orchard which reduces the heat loss by radiation to the night sky.
A small smudge
A small smudge
The plural of smudge is smudges.
A small smudge
Smudge isn't a Moshling.
The plural form of "smudge" is "smudges."
there was a smudge on the famous painting
Smudge