OF FREQUENCY OF THE BAROMETRIC HEIGHT AT DIVERS STATIONS. 439 
6. On the Standard Barometric Frequency Curve for the British Isles. 
In order to reach a fair appreciation of the manner in which the distributions of 
frequency differ at any two local stations, it is desirable to have a standard frequency 
curve with which either of them may be compared. This curve was obtained in the 
following manner. The mean values of a, p and y were found ; they are recorded 
with them standard deviations, probable errors and coefficients of variation below the 
“selected stations” in Table IV. Naturally these values did not satisfy the relation 
(p -r 1) = ya' , which is necessary if the curve is to be of the required type. 
Accordingly small alterations hq), Sy, 8a' were made in their values, proportional in 
each case to the corresponding probable error of qo, y, a, so that the relation 
+ Sp + 1) = (y + Sy) {a + Sa') 
was satisfied. The alterations in no case amount to 1 per cent, of the corresponding 
value, being 0’i79 of the probable errors of those values. The curve thus determined 
lias for its equation, the unit of y being a day, and of x one-tenth inch ;—■ 
y = 40-682 ( 1 ^-15-2364..^ 
the origin being at the mean height 29''-9221. This curve we shall in future speak 
of as the Standard Frequency Curve for the British Isles. It may be at first looked 
upon as an arbitrary curve, artificially obtained with a view to having some standard of 
comparison, but when it has once been plotted on the diagrams of the barometric 
frequency, we can give the standard curve a physical meaning. On examining 
Plate 15, fig. xvii. ; Plate 11, fig. vi. and Plate 12, fig. vii., it will be at once seen that the 
Standard Frequency Curve exjiresses the barometric frequency of places on a line 
running somewhat south of Scarborough, a trifle south of Llandudno, and slightly 
north of Parsonstown. Approximately we may state that the climate of Llandudno 
very nearly represents the standard of barometric frequency for the British Isles. 
Probably there would be hardly any sensible deviation at all between the standard 
frequency curve and the distribution of barometric frequency for places like Hull 
and Chester. 
A comparison of the mean frecjuency contour as sketched in on the map, page 424, 
with Plate 13 of the ‘Meteorological Atlas of the British Isles,’issued in 1883 by 
the Meteorological Office, shows that this contour is not very divergent from the 
isobar for 29''-90 ; it lies sensibly above the isobar given in that plate for 29''’92. 
The important suggestion now made is that a series of contour lines, generalised 
isobars, could be constructed, along which not only the mean barometric height would 
be the same, but practically all the constants which determine the distribution of 
barometric frequency. Our data are very far from sufficient to enable us to draw 
such a series satisfactorily, but an examination of Plate 10 will show that another such 
