he says,
shows the great power of perspiration. And though he does not pursue the
experiment, it is worthy of note as an attempt, like those of Janse
{128b} and others, to correlate the flow of water under pressure with the
flow due to transpiration.

It is interesting to find that Hales used the three methods of estimating
transpiration which have been employed in modern times--namely, (i)
weighing, (ii) a rough sort of potometer, (iii) enclosing a branch in a
glass balloon and collecting the precipitated moisture, the well-known
plan followed by various French observers.

He (_Vegetable Staticks_, p. 51) concluded his balance of loss and gain
in transpiring plants by estimating the amount of available water in the
soil to a depth of three feet, and calculating how long his sunflower
would exist without watering. He further concludes (p. 57) that an
annual rainfall of 22 inches is "sufficient for all the purposes of
nature, in such flat countries as this about Teddington."

He constantly notes small points of interest, _e.g._ (p. 82) that with
cut branches the water absorbed diminishes each day, and that the former
vigour of absorption may be partly renewed by cutting a fresh surface.
{129a}

He also showed (p. 89) that the transpiration current can flow perfectly
well from apex to base when the apical end is immersed in water.

These are familiar facts to us, but we should realise that it is to the
industry and ingenuity of Hales that we owe them. In a repetition (p.
90) of the last experiment we have the first mention of a fact
fundamentally important. He took two branches (which with a clerical
touch he calls M and N), and having removed the bark from a part of the
branch, dipped the ends in water, N with the great end downwards but M
upside down. In this way he showed that the bark was not necessary for
the absorption or transmission of water. {129b} I suspect that one
branch was inverted out of respect for the hypothesis of sap-circulation.
He perhaps thought that water could travel apically by the wood, but only
by the bark in the opposite direction.

Next in order (p. 95) comes his well-known experiment on the pressure
exerted by peas increasing in size as they imbibe water. There are,
however, pitfalls in this result of which Hales was unaware, and perhaps
the chief interest to us now is that he considered the imbibition of the
peas {130a} to be the same order of phenomenon as the absorption of wa