heedful

adj

1. Taking heed.
2. Paying close attention; mindful.

   “Hamlet: […]It is a damned ghost that we have seen, And my imaginations are as foul As Vulcan's stithy. Give him heedful note; For I mine eyes will rivet to his face, And after we will both our judgments join In censure of his seeming.”

   “As in decapitating the whale, the operator's instrument is brought close to the spot where an entrance is subsequently forced into the spermaceti magazine; he has, therefore, to be uncommonly heedful, lest a careless, untimely stroke should invade the sanctuary and wastingly let out its invaluable contents.”