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Dr. James Thomson then offered some remarks on the 

 smoky fog of Saturday evening, the 26th of November. 

 People in many cases at first thought the smoke pervading 

 their apartments was coming from some of the fires within 

 their own house ; and the truth subsequently became appa- 

 rent that Belfast was enveloped in a densely smoky fog. The 

 smoke remained most disagreeably evident, apparent to the 

 eyes, and perceived almost suffocatingly in the throat, even in 

 warmed apartments. What, then, could be the reason why 

 the air of the town should at that particular time, when the 

 mills were closed, be fifty-fold, or perhaps a hundred-fold, as 

 smoky as usual ? Mere calmness of the atmosphere would 

 not suffice to account for such excessive smokiness. He 

 thought the explanation of the phenomena in question was 

 afforded by a theory which he had submitted to the Natural 

 History and Philosophical Society in February, 1868. The 

 view he then gave, and which he thinks meets with confirma- 

 tion from the atmospheric phenomenon of the Saturday 

 and Sunday, was to the effect that the usual or general cause 

 of excessively smoky fogs is the arrival in the sky over a town 

 of a vast influx of warm air, much warmer than the air lying 

 calmly on the ground, and immediately over the houses. 

 The mingling of the two masses of air at different tempera- 

 tures must tend, according to well-known laws, to produce 

 cloud or mist at their junction, especially if both, although 

 clear, be nearly saturated with moisture in the gaseous state. 

 If the substratum of cold air be of considerable depth, the 

 first indications of the arrival of warm air may be a clouding 

 of the sky, and a rise of temperature at the ground, due to 

 radiation of heat downwards from the newly arrived air ; the 

 warmth at the ground often in such cases showing itself by an 

 incipient thaw, if there has been frost before. Then, as time 

 advances, the cold substratum may be gradually thinned away 

 by the scouring off of its upper part by the current of warm 

 air flowing above it, till the foggy junction where the inter- 



