This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
132. The danger of frosts, which are often so fatal to the blossoms of the Peach tree, obliges us, at the time of pruning, to retain more flowers than is absolutely necessary; and if the weather be favorable, too many fruits is the consequence. Fructification being a very trying process, the trees might be injured by its being prevented. Nevertheless, in years when only a moderate quantity sets, the thinning should not be made till the month of June, the time that the stone is formed, which is a crisis at which much fruit drops. When those that remain appear secure, the superabundant ones are removed, so as to leave only as many as the tree can bring to perfection, and nourish without exhausting itself. In this operaticn, the fruits that are too close together are thinned out, so as to distribute the whole equally, and as near as possible at uniform distances, giving the preference to those that are best placed, and of a regular form.
133. We first thin out those fruits that are at the tops of weak branches, or on branches of which the successional shoot appears weak; and there is always a less number left on the lower than on the upper parts, although the former have more flowers. The fruits to be removed must be detached by turning them with the thumb and the two first fingers, without jerking, taking care not to break off those intended to remain. When the growth of the tree is well balanced, the number of fruits left on each wing must be as nearly equal as possible; and if the thinning is well done, a sort of regularity is obtained which would make one believe that they had been placed on by hand. The green Peaches taken off may be turned to account by the confectioners. Notwithstanding the number of fruits dropped and thinned out, I still leave on each square-trained Peach tree, about four or five hundred Peaches, which, from their beauty and nearly equal size, well repay the trouble I take.
134. But in abundant seasons, if we did not thin till the stone is formed, the tree would be weakened. In such a case, the thinning should be made at two different times: the first in June, when all that are evidently superfluous must be thinned out; and the second after there is no danger of their dropping.
135. The greater or less quantity of fruit is a means of equalizing the strength of the different parts of the tree, as will be further explained.