In my free time today I've been coming back to the chapter you linked. I've now read through a good chunk of it.
Early on, Hirst demonstrates what little he thinks of the Regulative Principle. Here is a quotation (italics in original):
We are not aware that in the discourses of Jesus Christ, any explicit direction or commandment is put forth, as it respects their [musical instruments'] religious use. Certainly those who advocate their use, are justified in claiming the benefit of our Lord's silence. For it is fair to presume, that if their use be so repugnant to the spirit of the gospel, and so destructive of its operations, the great head of the church, who saw the conflicting opinions of distant ages as present, would have left on record some expressions of disapprobation... We say it might have been expected that if our Saviour had disapproved of the practice, in some of his explanatory discourses, or altercations with the Jews, he would have made us acquainted with the fact. And if these considerations will not avail, for want of our Lord's affirmative authority for instrumental music, vocal music will be in jeopardy for want of the same support. We gather from the sacred historians, that on one occasion, and but one, our Saviour with his disciples sung an hymn. In fact, it seems that the Redeemer did not intend to disturb the principle, or lawfulness of the practice in question at all, but to leave it open to the church, to follow or not, to a greater or lesser extent, as their circumstances, qualifications, and Christian judgment might decide.
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