On Mf/hf-Fyo.ot.<!. 55 



(lew can fall, and when it does, it is small in qnantity and consequently its 

 protective power is also veiy small. It instantly turns into hoar-frost, and 

 though it certainly. in tliat sliape. gives ont some heat. it is of very little 

 benetit to vegetation as niost of it évaporâtes. How sliglit the lielp is, which 

 Natuie reqnires in snch a case, we can judge from tlie foUowing calcnlation: 



At - 5" the air contains 0, 0034 kg. of water 1 



Ax ro ,v per m'. 



At +5 „ „ „ (),ooc8 „ „ „ j 



The différence, or 0,oo.t+ kg., must he snpplied to every nr. of air, in 

 order to raise the dew-point to + 5". This makes 340 kg. for 10 hectai-es, 

 while to raise the dew-point to + 1", it only requii'es 170 kg. of water. 



But on thèse exceptional occasions the dew-point rarely falls as low as 

 — 5,0, it usually stands higher, ahout — 2,,, and in that case no moi'e than 

 80 kg. of water are necessary foi' 10 hectares. 



If we sumniarize the observations made above, we have the followings 

 rules : 



The danger of frost is great if a calm comes on after dear, cold days, 



having a low tempei'ature. 

 The danger is greatei- in proportion to the dryness of the air, and great- 



est if the dew-point is below zéro. 

 The probability of frost can be judgcd from the colour of the sky after 



sunset; the more the blue tone prédominâtes, the greater is the 



danger — if the colour tends more towards grey, the danger is less. 

 If even a very slight motion in the air continues to within an liour, or 



an hour and a half before sunrise, no damage by frost need be feared. 



Now, it must be i-emembered that not every night which shows a tempé- 

 rature below O", brings us frost. As seen from the above observations, the 

 température of the four nights, the 11*'', 12"', 15*'', and 22'"' June, was 

 considei'abl}' below 0'^', and yet no damage was doue to the surrounding fields. 

 There was hardly a night between the 10*'' and 24*'' June when the tem- 

 pérature on thèse flelds did not sink below O", at least for a short period 

 and yet expérience shows us clearly that it was only on one night, the 

 21'' — 22'"* June, that there was any real danger to be feared, and of this we 

 had been forewarned by the usual signs. 



In conséquence of the slight breeze which continued towai'ds morning, 

 the danger of frost was not so great that the plants might have been damaged, 

 but it was evident that the result had hung on a hair. If the calm had come 



