OF FREQUENCr OF THE BAROMETRIC HEIGHT AT DIVERS STATIONS. 4G9 
and fall either straight into one of the tubes or on to one of the wedges, which will 
guide it into the corresponding tube. The shot will then be a record that the baro¬ 
meter needle stood once over the section corresponding to the tube. If balls be 
dropped in at regular intervals, a frequency record is obtained by simply counting up 
the number of balls in each tube. The record, of course, suffers from the disadvan- 
tao-e that the usual corrections cannot be made.'”' 
In practice, of course, the wedge edges d, d, d must be arranged so as to correspond 
exactly to the divisions between, say, the tenth inches. Any one tube will then 
record the number of times that the needle stood over a particular tenth when the 
record was taken. In the model itself a fixed gutter, ag, is brought over the centre 
of the needle gutter, just above its axis ; it stands clear of it, and runs back to a 
reserve of balls, R. A trigger arrangement, f,f, is fired by a clock-driven cam as 
often as desired. The trigger gear can be seen in the second figure. The large cam, 
C, turns round once in the twenty-four hours, one of the saw teeth being pushed on 
each hour by the hand of the clock. The channel R, containing the reserve of 
ballst in single file, is normally closed by the forked brass lever,//, which is pivoted 
near P, and bears at its lower end against the cam, being held forward by a spring. 
Only when the lever is fully cocked, pushed over to its furthest reach by the cam, 
does the fork of the lever stand in line with the reserve channel. One ball then 
drops into the fork, which is just of the thickness of a ball. When the lever is freed 
again, the fork comes opposite the fixed gutter {ci,g of fig. A), and the ball drops 
first into this, then falls into the needle gutter, and ultimately into a vertical tube. 
It may be desirable to take two or more frequency records for different hours of 
the day, and keep these records separate, so as to admit of the study of systematic 
differences. Mr. Horace Darwin, to whom the details of the working model are 
due, devised a very simple arrangement for doing this, which can be seen in the 
general view of fig. B. The balls do not drop straight from the needle-gutter into 
the collecting tubes, but run into radial gutters fixed to the dial-face of the liarometer. 
At their outer terminals these fixed radial gutters stand so far apart that they only 
deliver balls into alternate tubes, e.g., to the set marked on the figure with numbers 
not with black snots. Before the alternate records are made, the whole barometer, 
with the radial gutters attached, is given a slight turn by the same clock that serves 
to release the trigger. The radial gutters now stand opposite the tubes marked with 
black spots, and the record is made in them. 
The working model was kept running for rather more than a month, and worked 
very satisfactorily, considering its somewhat rough construction. There were one or 
two “ misfires,” due to the balls hanging up, but such accidents could be easily 
remedied by a slight alteration. 
* It must be remembered, however, that grouping in tenths of an inch gives quite sufficiently smooth 
and detailed returns for the purpose of calculating frequency curves. 
t Bicycle bearing balls were actually used, as shot were found too irregular in shape and size. 
