88 The N.Z. Journal of Science and Technoloo-y. [Mar. 
from coastal stations, state of sea and tide—are all taken simultaneously. 
The most important of these reports is the barometer-reading, and in order 
that the observations from this instrument may be intercomparable they 
must be corrected and reduced to sea-level. This is necessary because the 
pressure of the atmosphere diminishes with height, and so the reading 
of the barometer decreases approximately one-tenth of an inch for every 
100 ft. increase in altitude. The corrected readings are entered by signs 
and figures on a plain map, and lines are then drawn connecting all those 
stations having the same barometric pressure. These lines of equal pressure 
are called isobars , and are drawn for every tenth of an inch : thus a line 
would join all places representing a pressure of 29*9 in., 30-0 in., 30*1 in., 
and so on. 
The wind is indicated by arrows, the head of the arrow pointing in the 
direction in which the wind is blowing, and the force being shown by 
arrows of different types. An arrow with a barb on one side only indicates 
light wind (see fig. 1), a barb on each side indicates a breeze, while one, 
two, and three “ feathers ” on one side of the shaft indicate progressively 
stronger winds, the last representing a gale. There are also numerous 
signs representing different conditions of weather, state of sea, &c. 
Ballot's Law. 
If the different synoptic charts which follow are examined definite prin¬ 
ciples will be perceived in regard to the direction of the wind with relation 
to the isobars. Professor Buys Ballot, of Utrecht, was the first to formulate 
a general rule with reference to the direction of the wind and its relation 
to barometric pressure. This was in 1850, the law in simple language 
being : “ Stand with your back to the wind and the barometer will be 
lower on your right hand than on your left.” This is applicable in the 
Southern Hemisphere, while in the Northern left must be substituted for 
right, and right for left. For example, suppose at Wellington a northerly 
wind is blowing, then if we face south our right will be the side where the 
barometer is lower ; if the wind be westerly, then the barometer should be 
lower to the south and higher to the north of us. The wind does not assume 
a direction strictly parallel with the isobars, but it is nearly always found 
that the direction is inclined at an angle of about 35° to the isobar and 
towards the position of low barometric pressure. 
Mountain-ranges, steep coast-lines, or other topographical features often 
cause a marked deflection in the wind-direction, in some cases to such an 
extent that the very opposite wind may be experienced to what would be 
anticipated according to the isobaric trend. 
Baric Gradient and Wind-velocity. 
From the consideration of the direction we now pass on to that of the 
velocity or rate of the wind, which is likewise related to the distribution 
of atmospheric pressure. If we take a line at right angles joining two 
isobars, the measure of this line is called a “ gradient.” The shorter the 
distance between the isobars the greater or steeper is the baric gradient, 
just in the same way that the varying distances between contours on a 
military map denote varying steepness of the hills. It is the practice to 
compute the gradient in hundredths of an inch per 15 nautical miles, or 
the quarter of a geographical degree ; but when one becomes accustomed 
to the drawing of isobars over a certain defined area one very quickly 
becomes experienced in estimating the amount of gradient approximately 
enough for practical purposes. The barometric gradient enables one to 
.anticipate the velocity of the wind, the law applying to this being that 
