Effects of Hoar-Frost m Plants. 141 



tlie thermometer, in the parish of Keig, stood at 5° below zero. I am also 

 unable, to furnish any experimental observations on the moisture of the atmo- 

 sphere; but, considering that the district is environed by hills, and that much 

 of the rain which falls will naturally find its way to the plains, we may believe 

 the air to be rather of a humid description. 



These observations on temperature and moisture, lead me to remark an evil 

 connected with these circumstances, which takes place in the valley of Alford, 

 to an extent not frequently paralleled in other places ; I mean the injury 

 which the crops sustain from hoar-frost, in the end of summer, or the early 

 part of autumn. This takes place, for the most part, in low situations ; and 

 as these lie, in general, near the river and marshy ground, and are themselves 

 often wet, the injury is usually ascribed to a cold vapour rising up from the 

 river and wet places. As far as the river is concerned, the opinion is un- 

 doubtedly without foundation ; for fields in the neighbourhood of many other 

 rivers suffer no damage of this kind : and, farther, the injury only takes place 

 when there is no wind which could waft the vapour from the river to the ad- 

 joining grounds. It is, besides, well known that vapour never rises to a 

 great extent, except when a current of air is passing over the evaporating sur- 

 face ;"but, during wind, as I have mentioned, the frosting of crops never takes 

 place. A crop, indeed, upon a wet soil, may be apt to suffer from hoar-frost, 

 in so far as the atmosphere over it is, during a calm, perhaps more loaded 

 with vapour than in other situations ; and, consequently, more calculated for 

 depositing dew and hoar-frost. A crop also, near a wet grassy situation, may 

 be injured in this way. Surfaces of that kind are well known to throw off, in 

 clear nights, their heat very readily ; and the coldness thus produced will be 

 communicated to fields in the neighbourhood. I doubt, however, whether the 

 state of the ground as to moisture has any material influence over the hoar- 

 frost, and am rather disposed to explain all the circumstances upon the prin- 

 ciples laid down by Dr Wells and others, as regulating the formation of dew 

 and hoar-frost. In a cloudless and perfectly calm night, the earth throws oflT 

 the heat it had received during the day, in such a manner, that its surface, as 

 well as the air, for several feet upwards, becomes often 10° or 12°, and some- 

 times 15* colder than the rest of the atmosphere ; and the formation of dew or 

 hoar-frost naturally takes place. Farther, it is known that a flat or horizon- 

 tal surface loses more heat, and therefore becomes more loaded with dew or 

 hoar-frost, than a similar surface, placed vertically, or with an inclination. In 

 short, it is established, that a clear sky, a dead calm, and a horizontal sur&ce, 

 are the circumstances most favourable,— the two first being perhaps absolute- 

 ly necessary, — to the formation of dew or hoar-frost ; and these facts go far, I 

 think, to explain the damage sustained by crops in the valley of Altbrd from 

 hoar-frost. They shew why, in that situation, the lowest land, always more 

 or less sheltered from the wind, and having frequently a horizontal aspect, is 

 damaged ; while the higher ground, exposed oflen to a current of air, when 

 not a breath is stirring in the lowest situations, and which, besides, have usual- 

 ly an inclined surface, receive no iiyury. They shew also why the valley of 

 Alford, surrounded by hills, and in that way sheltered, suffers more than other 

 valleys and plains. In larger valleys, the hills must have less influence in re- 

 straining the currents of the atmosphere ; and, in very small valleys, the air 



