Mr. Slee on the Helm Wind of Cross Feil. Ald 
enables persons situated beyond the limit of the current, to pro- 
nounce with certainty on its existence. The current is strongest 
near to the summit of the mountain ; and on the eastern side of it, 
nothing more is felt than a slight breeze. 
It ought to be particularly remarked that the Helm never has 
taken place excepting when the direction of the wind is easterly ; 
but there does not appear to be the slightest connection between 
the velocities of the general and the partial currents ; for, as I have 
before stated, the Helm may be raging with great violence, whilst 
at the distance of eight miles scarcely a breath of wind can be felt: 
I have not yet had an opportunity of observing how the barome- 
ter is affected by the action of the current near to the base of the 
mountain, though I am exceedingly desirous to make the experi- 
ment, and for the following reason. In the first place it is well 
known, and I have ofteu verified the fact, that when currents of 
air are passed rapidly over surfaces, instead of pressing upon them, 
as might be expected from reasoning on the subject, they actually 
diminish the atmospheric pressure, and exert, if I may be allowed 
the expression, a negative normal force. ‘The experiment of the 
two cards, which has of late attracted the attention of scientific 
men, shews in a very striking manner the action of this force; and 
were this phenomenon explained, it would evidently account also 
for the fall of the mercurial column when a current is passed over 
the surface of the fluid in the cistern. 
_ The reasoning which Professor Leslie has employed in the Sup- 
plement to the Encyclopedia Britannica, to account for the fall of 
i ales during high winds, is in this case totally inappli- 
cable. 
Suppose, then, a barometer placed at the bottom of Cross Fell 
before the commencement of the Helm ; what will be the conse- 
quence when the current begins to blow? We know that the par- 
ticles of air will impinge obliquely on the mercury in the cistern, 
and the force of each may therefore be resolved into two others, 
one perpendicular and the other parallel to the horizon: the first, 
of course, will tend to raise, and the second, from what has just 
been stated, will tend to depress the column in the tube. The 
question is therefore reduced to this, viz. which of these forces will 
predominate ? The answer can, in my opinion, only be determined 
by experiment, and I hope ere long to be able to ascertain the 
point by this means. There is usually, but not in all cases, a sort 
of drizzling rain in the vicinity of Cross Fell during the occurrence 
of the Helm, and this leads me to suspect that the barometer stands 
low at the time. 
I have no theory to offer, by way of explaining the Helm, inas- 
much as some of the facts relative to it, appear to me hardly com- 
patible with the laws of matter and motion. Such, for instance, as 
the perfect repose of the Bar where the current is strongest ; such 
also is the very circumscribed limits within which it exerts its ac- 
