Mineralogy, Medicine, <^r. 421 



free of expense for carriage, to the Sccietaiie Gentrale, before 

 the 31 October, 1820, The prize is 300 francs. 



§ Meteorology, Magnetism, &c. 

 1 . — On the Cold vjhich occurs at Sun-rise, and on Nitre in Plants. 



Oxford, 10 April, 1820. 



Sir, — I observe in your last Number an allusion to the cold 

 usually experienced in India, about the time of sun-rise. As 

 this phenomenon, or something very nearly resemblino it, 

 takes place in this country also, and, as far as I have been 

 able to ascertain, in all others, it has often been a matter ol 

 surprise to me that its causes should have been little, if at sAl, 

 investigated. The Meteorological Diary which appeals in 

 your Journal, and all that I have ever seen, state the height of 

 the thermometer at a certain fixed hour, without any regard to 

 the situation of the sun : now it appears to me that an accurate 

 account of the thermometer at a certain period of the day, 

 (viz., the morning dawn,) and a comparison of its indications 

 then and at noon, or some fixed hour, or at one hour before 

 and one after, would tend to throw some light on the causes of 

 the phenomenon in question. It is one to which I have paid 

 considerable attention for a good many years, and have never 

 been able to obtain a satisfactory solution. It is very striking, 

 and has been noticed by many ; but their observation of it 

 being in general very limited, they, for the most part give an 

 incorrect, because a partial, description of it ; e. g. some 

 speak of a certain fixed hour as the coldest in the twenty-four: 

 this is true during, perhaps, about one month in the year, 

 and no longer, viz., when that hour happens to coincide with 

 the day-break: the case is much the same with those who fix 

 upon half an hour before sun-rise ; which may or may not 

 coincide with the point in question. 



The phenomenon, according to the best observations I 

 have been able to make, and which I think will be confirmed by 

 the regular inspection of a thermometer, is this : at the first 

 dawning of morning twilight, or within twenty minutes at the 

 utmost, of that point, a sudden diminution of temperature 



