1892 - 93 .] Mr Aifcken on Hazing Effect of Atmos'plieric Dust. 87 
tion of the N.E. and S.W. winds, all the others show an increase 
in the limit of visibility with increase of dryness. In the two excep- 
tions referred to, each has only one figure not in accordance with the 
law, and both of these only show a very slight deviation from it. 
In place of using the figures in Table No. lY. for drawing the 
isatmid lines, perhaps a more correct result might have been 
obtained by manipulating the figures still further, either by plotting 
on paper the limit of visibility for each direction of wind for increas- 
ing dryness, and drawing an easy curve through the points, and using 
the limits so obtained for drawing the isatmid lines. No doubt in 
this way a more finished set of isatmid lines would have been ob- 
tained, and the Plate would have been more pleasant to look at ; but 
it is not thought that the number of observations is sufficient to 
justify the finished and final appearance which this process would 
give to the curves. The curves have tlierefore been drawn from 
the figures as they stand in Table No. IV., and they correctly indi- 
cate an unfinished state of matters. 
It may be a,s well to point out the difference between these isatmid 
lines and isobaric or isothermal lines. While the latter repre- 
sent the position of uniform pressure or temperature at a particular 
hour, the former do not represent the condition at any particular 
time ; in fact, they represent conditions which never have an exist- 
ence at one and the same time. An isatmid line for any particular 
hour would be a circle of a certain radius, described round the place 
of observation as a centre, on the supposition that no change took 
place in the air from the time it entered within the circle till the 
time it left it. In order, further, that the isatmid line should be 
a circle, we would also require to neglect the position of the sun 
relatively to the direction of observation. The isatmid lines repre- 
sent the transparency of the air, not at any particular time, but of 
the different winds at different degrees of dryness. 
The observer is supposed to be standing at Falkirk and to be 
looking directly towards the. point from which the wind is blowing, 
and the isatmid lines show the mean limit of visibility for air 
coming from that direction at different humidities. Suppose, for 
instance, that at the time the wind was from the N.W., and that 
the wet bulb showed a depression of between 3° and 4°. Then 
the point where the line of vision — that is, a line drawn N.W. — is 
