What is "instrumental errors"

Instrumental \In`stru*men"tal\, a. [Cf. F. instrumental.]

1. Acting as an instrument; serving as a means; contributing to promote; conductive; helpful; serviceable; as, he was instrumental in conducting the business.

   The head is not more native to the heart, The hand more instrumental to the mouth.
   --Shak.

2. (Mus.) Pertaining to, made by, or prepared for, an instrument, esp. a musical instrument; as, instrumental music, distinguished from vocal music. ``He defended the use of instrumental music in public worship.''
   --Macaulay.

   Sweet voices mix'd with instrumental sounds.
   --Dryden.

3. (Gram.) Applied to a case expressing means or agency; as, the instrumental case. This is found in Sanskrit and Russian as a separate case, but in Greek it was merged into the dative, and in Latin into the ablative. In Old English it was a separate case, but has disappeared, leaving only a few anomalous forms.

   Instrumental errors, those errors in instrumental measurements, etc., which arise, exclusively from lack of mathematical accuracy in an instrument.