But again--you have your _sun_-measure, and can tell exactly at any moment how strong, or how weak, or how wanting, the sun is. But the sun-measurer can't tell you whether the rays are stopped by a dense _shallow_ cloud, or a thin _deep_ one. In healthy weather, the sun is hidden behind a cloud, as it is behind a tree; and, when the cloud is past, it comes out again, as bright as before. But in plague-wind, the sun is choked out of the whole heaven, all day long, by a cloud which may be a thousand miles square and five miles deep.
And yet observe: that thin, scraggy, filthy, mangy, miserable cloud, for all the depth of it, can't turn the sun red, as a good, business-like fog does with a hundred feet or so of itself. By the plague-wind every breath of air you draw is polluted, half round the world; in a London fog the air itself is pure, though you choose to mix up dirt with it, and choke yourself with your own nastiness.
Now I'm going to show you a diagram of a sunset in entirely pure weather, above London smoke. I saw it and sketched it from my old post of observation--the top garret of my father's house at Herne Hill. There, when the wind is south, we are outside of the smoke and above it; and this diagram, admirably enlarged from my own drawing by my, now in all things best aide-de-camp, Mr. Collingwood, shows you an old-fashioned sunset--the sort of thing Turner and I used to have to look at,--(nobody else ever would) constantly. Every sunset and every dawn, in fine weather, had something of the sort to show us. This is one of the last pure sunsets I ever saw, about the year 1876,--and the point I want you to note in it is, that the air being pure, the smoke on the horizon, though at last it hides the sun, yet hides it through gold and vermilion. Now, don't go away fancying there's any exaggeration in that study. The _prismatic_ colors, I told you, were simply impossible to paint; these, which are transmitted colors, can indeed be suggested, but no more. The brightest pigment we have would look dim beside the truth.
I should have liked to have blotted down for you a bit of plague-cloud to put beside this; but Heaven knows, you can see enough of it now-a-days without any trouble of mine; and if you want, in a hurry, to see what the sun looks like through it, you've only to throw a bad half-crown into a basin of soap and water.