410 Proceedings of the Royal Society 
The remaining Botanical inquiry I have to notice is one well 
known to naturalists, and of more general interest than the pre- 
ceding. Great doubts had been long entertained among good 
authorities about the nature of the motion of the sap in trees. Dr 
Walker, Professor of Natural History in the University of Edin- 
burgh, undertook a series of well-devised and precise experiments 
to determine how the sap moves in trees in the spring; and having 
with philosophical caution repeated them in several successive 
years, with the same results, he communicated the whole inquiry 
to the Eoyal Society in 1783 and 1785. This inquiry is still held to 
be authoritative proof, that the movement of the sap in trees, on the 
arrival of genial weather in the spring, is not a movement of circu- 
lation, by ascent and descent, as many had before contended, but 
invariably a simple movement of ascent ; that the sap ascends 
neither in the pith, as some had maintained, nor in the bark, as 
insisted on by others, but in the wood, and between the wood and 
bark ; that the date of its commencement, and its rate, both depend 
on the earliness and geniality of the warm season ; that the ascent 
varies in rate, from six to nine inches daily, according to the pre- 
vailing temperature of the air ; and that those buds always open 
first into leaves which the sap first reaches, so that its arrival is the 
essential cause of their growth. The author, however, points out 
that his experiments fix only the nature of the motion of the sap 
in the spring, when the tree has no leaves, and takes no account of 
what the movement may be when the leaves are developed. He is 
inclined, indeed, to presume that it may then be different; and 
accordingly ulterior inquiries have shown that, when the tree is in 
leaf, the sap moves downwards as well as upwards, observing now a 
circulation. 
In Meteorology, if a man be only a good looker, he may one 
day become an original observer. It is, therefore, a favourite study 
with those fond of Natural Science. Accordingly, we find upwards 
of twenty communications, and not a few of them very valuable, 
on the subject of meteorology in the Society’s early Proceedings. 
Playfair in 1784: acutely investigates the causes which affect the 
accuracy of Barometric Observations. In 1790 Dr Rutherford 
describes a self-registering thermometer, by which the maximum 
