274 PROFESSOR C. PIAZZI SMYTH ON 



(Table XXV.) But towards this end, Table XXV., — which is of double extent, has, in 

 terms of the same day-degrees of botanical temperature, an accumulative character as well ; 

 and goes on adding up in separate columns, from the beginning to the end of the year, 

 month by month, the negative day -degrees in one column, and the positive day-degrees 

 in another. 



(Table XXV. cont.) Whence we may see at once for either any year, or for the mean 

 of all the years, how much negative or repressive effect, vegetation had to withstand during 

 the earlier part of the year ; then how much positive and encouraging effect it received 

 during the middle of the year ; and finally by how much increase of cold, combined with 

 duration of the same towards the end of the year, the growing powers of plants were 

 sealed up again in winter sleep. 



(Table XXV. cont.) On an average of all the years the repressive effects of the beginning 

 of the year have exhausted themselves in the first three months, after scoring 341 day- 

 degrees of botanical cold. Then begin and increase through the next seven months, or 

 April to October, both inclusive, the positive day-degrees of botanical life and growth, 

 scoring at last 2070 of such degrees. While after that, begins the cold of November and 

 December, adding 192 day-degrees of cold to the Spring's 341°; or making the repression 

 = 533 day-degrees of cold for the whole year ; and leaving the positive effects of summer 

 in excess by 1537 of such degrees. Out of which number, both Nature and the farmers 

 of Scotland have to get all their annual living as they best can. 



(Table XXVI.) Both Table XXIV. previously described with its statical quantities, 

 and Table XXV. last described with its quasi dynamic and accumulative ones, are often at 

 variance with themselves, on contrasting one year with another, on account largely of the 

 variability of our climate. So that in some years, some amount of positive botanical tem- 

 perature begins in March ; but is immediately followed by so much botanical cold in April 

 and May, that the true spring of vegetation for the year only begins in June. Where- 

 fore the false start it received so early in March, is a disastrous thing for the plants, and 

 typifies an unfavourable, rather than a favourable, agricultural year. Hence a better 

 notion of when the growing period really begins, may be obtained from the present Table 

 XXVI. , now to be described. It is a cumulative Table also, and of botanical day-degrees; 

 but has its summations made up and combined in one monthly column only, and according 

 to algebraic sign. The negative symbol therefore of the resultant, only changes into the 

 positive, when the latter has, in a manner, eaten up the former : and the dates so obtained 

 for spring-tide, are later than what would be derived from the first Table, viz. Table 

 XXIV., but far more assured and practical. 



(Table XXVI. cont.) Hence by Table XXVI. Plant-growth, as shown by the negative 

 quantities at the beginning of the year changing into positive, begins on the average of 

 Scotland soon after the middle of May. But there have been years when it did not begin 

 until the end of June, and others again when it fairly began before the end of April. 



(Table XXVI. cont.) In a similar manner the growing and ripening period in Autumn, 

 if held to be defined by no more accessions of botanical heat, ends on the average with 



