A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 19:1After exhibiting the harmonious revelation of God's perfections made by His works and His word, the Psalmist prays for conformity to the Divine teaching. (Psa 19:1-14)
the glory of God--is the sum of His perfections (Psa 24:7-10; Rom 1:20).
firmament--another word for "heavens" (Gen 1:8).
handywork--old English for "work of His hands."
psa 19:2uttereth--pours forth as a stream; a perpetual testimony.
psa 19:3Though there is no articulate speech or words, yet without these their voice is heard (compare Margin).
psa 19:4Their line--or, "instruction"--the influence exerted by their tacit display of God's perfections. Paul (Rom 10:8), quoting from the Septuagint, uses "sound," which gives the same sense.
psa 19:5The sun, as the most glorious heavenly body, is specially used to illustrate the sentiment; and his vigorous, cheerful, daily, and extensive course, and his reviving heat (including light), well display the wondrous wisdom of his Maker.
psa 19:7The law is described by six names, epithets, and effects. It is a rule, God's testimony for the truth, His special and general prescription of duty, fear (as its cause) and judicial decision. It is distinct and certain, reliable, right, pure, holy, and true. Hence it revives those depressed by doubts, makes wise the unskilled (Ti2 3:15), rejoices the lover of truth, strengthens the desponding (Psa 13:4; Psa 34:6), provides permanent principles of conduct, and by God's grace brings a rich reward.
psa 19:12The clearer our view of the law, the more manifest are our sins. Still for its full effect we need divine grace to show us our faults, acquit us, restrain us from the practice, and free us from the power, of sin. Thus only can our conduct be blameless, and our words and thoughts acceptable to God.