Note 1: (consumptive) Consumption is known in the modern world as tuberculosis.
Note 2: (Cowper's Tirocinium) The poem refered to here urges a friend of William Cowper not to send his son away to school, but instead, to educate him at home. Part of the reasoning given is that the separation is both unnecissary and damaging to the natural affection between father and son. Here is a section of the poem, and one which contains the line used by Jane Austen:
Why hire a lodging in a house unknown
For one whose tenderest thoughts all hover round your own?
This second weaning, needless as it is,
How does it lacerate both your heart and his!
The indented stick, that loses day by day,
Notch after notch, till all are smoothed away,
Bears witness, long ere his dismission come,
With what intense desire he wants his home.
But though the joys he hopes beneath your roof
Bid fair enough to answer in the proof,
Harmless, and safe, and natural, as they are,
A disappointment waits him even there:
Arrived, he feels an unexpected change;
He blushes, hangs his head, is shy and strange
No longer takes, as once, with fearless ease,
His favourite stand between his father's knees,
But seeks the corner of some distant seat,
And eyes the door, and watches a retreat,
And, least familiar where he should be most,
Feels all his happiest privileges lost.
Alas, poor boy!-the natural effect
Of love by absence chill'd into respect.
~ William Cowper
The poem in its entirety is available here: Tirocinium and is well worth reading. Many of the themes in the poem echo ones in Mansfield Park, particularly the themes of parental responsibility and the moral education of children.
For more information on the concurrence of themes between Tirocinium and Mansfield Park, click here.
Note 3: (foolish precipitation last Christmas) Mary is referring here to Edmund's ordination as a minister in the church, which she continues to denigrate as "foolish", and "evil", and a "stain" to be blotted out.