"From the latter end of January to the middle of May trees will bleed. Those that run first, are the Poplar, Asp, Abele, Maple, Sycamore. Some, as Willows and the Birch, are best to tap about the middle of the season, and the Walnut towards the latter end of March.
"When a large Walnut will bleed no longer in the body or branches, it will run at the root, and longer on the south or sunny side than on the north or shady side.
"A culinary fire will have the same or greater effect than the sun, and immediately set trees a-bleeding in the severest weather. Branches of Maple or Willow cut off at both ends, will bleed and cease at pleasure again and again as you approach them to or withdraw them from the fire, provided you balance them in your hand, and often invert them to prevent the falling and expence of the sap; but at length they cease.
"A Birch will not bleed however deeply the bark only may be wounded: it is necessary to pierce into the substance of the wood."—Dr. Tonge in Phil. Trans. No. 43.