On Meteorological Observations. 91 



so-called radiation fogs to which we are rather liable. Now 

 the position of Croydon is such as to receive the united streams 

 of cold air from the Caterham Valley, Smitham Bottom, and 

 the coombes leading into them, and we may accordingly expect 

 to find, and do find, great difference of temperature within 

 short distances on certain occasions in Croydon. The descent 

 of cold air in this way through the valleys is of course counter- 

 balanced by the ascent of the comparatively warm air which it 

 displaces, so that the hills enjoy a somewhat higher tempera- 

 ture than they otherwise would. 



Take your stand some calm and cloudless summer evening 

 after a hot day on the mound by the water-tower, and look 

 over the country to the south and west. As the sun sinks you 

 will notice that the smoke of South Croydon is less diffused 

 through the air than it was an hour before, and by-and-bye, as 

 the twilight deepens, it will settle down in a well-defined 

 stratum, extending up the valley to Caterham Junction, reaching 

 perhaps half-way up Russell Hill, and leaving the air trans- 

 parent above. The outline of the hills ie sharply defined 

 against the evening sky, and their shoulders loom darkly above 

 the haze in the valley. The roar and rush of a train descending 

 the cutting from Merstham tunnel perhaps falls on the ear in 

 measured cadences. All else is still save the hum and murmur 

 of the town below. The air is soft and balmy with the scent of 

 flowers, and you enjoy the freshness. Now descend Park Hill 

 and cross the meadow by the footpath to Coombe Lane. Just 

 before you reach the wicket-gate through the hedge, at the 

 bottom of the dip you will suddenly find it chilly and cold, and 

 very likely a faint breeze from the east coming down the hollow 

 from Coombe Farm will fan your brow. A few paces farther 

 on, at the top of the rise, you will again come upon the 

 warm and genial summer air. Suppose you carry a ther- 

 mometer you will find its indications respond to your feelings, 

 and that it is several degrees colder in the hollow than else- 

 where. You will encounter similar phenomena, but less 

 marked, by descending from the water-tower to the Brighton- 

 road and climbing the hill opposite ; you will cross a cold current 

 of air setting slowly down the valley to Croydon, and leave it 

 behind on emerging from the haze. 



Under similar circumstances of clearness of the sky and 

 stillness of the atmosphere this phenomenon is very marked 

 when the ground is wrapped in a mantle of freshly fallen snow, 

 for then the heat of the ground is effectually cut off by one of 

 the best non-conductors in nature — a layer of snow with air 

 entangled in its interstices, and the cooling effect of radiation 

 is uttchecked, A remarkable instance of this came under my 

 notice on the 23rd of November, 1879. The ground was 



