372 MILDEWED WHEAT. 



tive the disease is to that grain, as well as to others visited 

 by it. Many growing crops and bundles of straw of the pre 

 vious year s growth were examined, and the result of my 

 observations tended to strengthen the opinions I had formed 

 of the origin and localities of its effects. 



Mildew appears to me to result from frost, produced by the 

 radiation of heat, rupturing the sap-vessels, and the moisture 

 which exudes being favourable to the germination of the 

 seeds of the fungus which grows on the straw of the plant, and 

 checks the filling of the grain. It has been established by 

 repeated experiment, that in certain states of the atmosphere, 

 cold, within the limits of freezing, takes place on the surface 

 of the earth when the temperature, at an elevation of a few 

 feet, is ten or twelve degrees warmer. This is beautifully 

 explained in &quot; Wells Essay on Dew,&quot; which I recommend to 

 farmers wishing to become acquainted with atmospheric effects 

 on vegetation. The natural agency favourable to the radia 

 tion of heat, or production of cold, is a clear sky and still 

 atmosphere. The luxuriance of crops is a predisposing cause 

 to a visitation of mildew, from the breadth, colour, and succu- 

 lency of the foliage. 



In the year 1830, I made an attempt to prove the correct 

 ness of my opinions regarding mildew by experiment. My 

 apparatus, which was of the simplest kind, was often exposed ; 

 but the difficulty of catching a favourable atmosphere rendered 

 all my attempts unsatisfactory ; but some of my observations 

 with the thermometer were remarkable. On the 19th August, 

 at eight o clock in the evening, a delicate thermometer, on 

 Fahrenheit s scale, at four feet from the ground, indicated 

 45 ; and a similar one, immediately below the other, exposed 

 on the surface of the grass, 38 ; and at half-past eight, re 

 spectively, 47 and 43, the wind having risen in the interval. 

 Next morning, at four o clock, the thermometers stood at 

 38 and 35 ; and another, enclosed in a glass-case, and ex 

 posed on the outside of a window, at 45. At five o clock, 

 the thermometer, four feet from the ground, indicated 34, 

 and the one on the grass, 301. The thermometer at the win 

 dow remained unaltered, and, being removed from the case, 

 -was placed with the other two on a piece of lodged spring- 



