104 Proceedings of the Royal Society of Edinburgh. [Sess. 
of the advance of a well-developed cyclonic depression from the westward. 
It has long been recognised, by seamen and other observers of weather, in 
observations of upper clouds which are seen to be moving from the north- 
west while the surface winds are coming from the south-west. It is one 
of the surest signs of the rainfall which occurs in the front of a cyclonic 
depression. The table already given shows the values of A N p and A w p for 
each kilometre level, and the values of A^d and Awd computed from the 
changes in the pressure-differences for successive kilometre steps. We 
may note here an ambiguity of notation, which we ought to find some 
means to remove, and which ought at least to be made clear. In the table 
A p and A# are used to indicate the slope of pressure and of temperature in 
the two directions N. and W. Thus in the table, when A p or Ad is positive 
for a given direction, it is to be understood that it represents the fall of 
pressure in that direction. But the usual convention of the differential 
calculus is that an increase in the quantity represented is indicated by a 
positive value of the difference. The ambiguity arises from the use of the 
convenient symbol A to denote the difference, while the meteorological 
practice is to think of gradient as represented by downward slope. I have 
not found any convenient new symbol to use instead of A to indicate a 
negative difference, so the ambiguity remains for the present, though I feel 
that an apology is due for it. 
In order to present in a table the corresponding values of A p and Ad 
for the same level, I have taken the means of the two values of A p for the 
top and bottom of the kilometre to which Ad refers. This practice is, 
perhaps, rather doubtful, but except in Table VI. it has been followed in 
the tables already given, so I adhere to it in this one. 
Converting by simple inversion the figures for Ap and Ad per 100 
kilometres into distances along the axis of the intercepts of the next higher 
isobar and isotherm respectively, we obtain the following : — 
Table VIII. 
Height. 
Distance of next higher 
isobar in kilometres. 
Distance of next higher 
isotherm in kilometres. 
k. 
k. 
k. 
k. 
k. 
5-6 
84 S 
143 W 
60 S 
102 W 
4-5 
109 S 
263 W 
64 S 
50 W 
3-4 
141 S 
2000 W 
135 S 
132 W 
2-3 
131 S 
526 E 
244 N 
125 W 
1-2 
141 S 
312 E 
93 S 
909 E 
0-1 
200 S 
232 E 
270 S 
222 W 
