ocx> — mar. 1858-59.] Progress of the Magnetic Survey. 835 



There was quite regularly a hoar-frost setting in, even if the mi- 

 nimum temperature of the air was 2 or 3 degrees above the freez- 

 ing point. The hoar-frost was produced by the great radiation of 

 the leaves and grass during the serene nights. 



At Umerkuntunk itself I observed some remarkable irregulari- 

 ties of temperature. Considering the cold 

 p^^^^Umerkm- temperatures experienced in the stations 

 tuk. of the Nerbudda valley, Westward of Umer- 



kuntuk, it might be expected to find the minimum temperature 

 of the night at Umerkuntuk below zero ; but on the contrary the 

 minimum temperature of the air on the plateau, at the end of 

 January, was constantly 9 degrees and even 12 degrees centi- 

 grade above the freezing point. It was therefore considerably 

 warmer than the minimum of the valleys at the foot of the plateau. 

 Besides the minimum does not take place just before sun-rise but 

 one, or one-and-a-half hour earlier. The reason of this phenome- 

 non is that during the night, the cold air, which is heavier, flows 

 down along the sides of the hills, and accumulates at the bottom 

 of the valleys, whilst at the top of the small plateau itself, 

 the cold air, which is locally formed by the radiation of leaves and 

 grass, is immediately carried away by the winds into the surround- 

 ing atmosphere and re-placed by somewhat warmer air. 



During the day-time the temperature at the top of Umerkuntuk, 

 is, of course, much lower than in the valleys below. During my 

 stay there the highest observed temperature was 75 degrees Fah- 

 renheit, exceeding by 5 degrees Fahrenheit the maximum tempera- 

 ture of all the other days. 



There was only one day quite free of clouds during my stay at 

 Umerkuntuk. Generally, soon after 8 o'clock a. m., quantities of 

 cirri dispersed all over the sky, and mitigated the full power of 

 the sun's rays : they only dissolved themselves partially in the 

 evening before sun-set, some portions remaining even during the 

 night. 



I had some rain on the 29th and 30th of January, it appeared 

 to have extended over a large area, and greatly affected the tem- 

 perature of the air, cooling it down so, that the maximum tem- 

 perature was lowered more than 5 degrees. 



