MEAN TEMPEHATUEE. 
493 
European coni cultivated in large quantities in so low a 
region. The fine fields of wheat in Mexico are between six 
hundred and twelve hundred toises of absolute elevation; 
and it is rare to see them descend to four hundred toises. 
We shall soon perceive that the produce of grain augments 
sensibly, from high latitudes towards the equator, with the 
mean temperature of the climate, in comparing spots of 
different elevations. The success of agriculture depends on 
the dryness of the air; on the rains distributed through 
different seasons, or accumulated in one season; on winds 
blowing constantly from the east; or bringing the cold 
air of the north into very low latitudes, as in the gulf of 
Mexico; on mists, which for whole months diminish the 
intensity of the solar rays ; in short, on a thousand local 
circumstances which have less influence on the mean tempe- 
rature of the whole year than on the distribution of the 
same quantity of heat through the different parts of the year. 
It is a striking spectacle to see the grain of Europe culti- 
vated from the equator as far as Lapland in the latitude of 
69°, in regions where the mean heat is from 22° to — 2°, in 
every place where the temperature of summer is above 9 3 or 
10°. Wo know the minimum of heat requisite to ripen 
wheat, barley, and oats ; but wo are less certain in respect 
to the maximum which these species of grain, accommo- 
dating as they are, can support. We are even ignorant of 
all the circumstances which favour the culture of corn 
within the tropics at very small heights. La Victoria and 
the neighbouring village of San Mateo yield an annual 
produce of four thousand quintals of wheat. It is sown in 
the month of December, and the harvest is reaped on the 
seventieth or seventy-fifth day. The grain is large, white, 
and abounding 'n gluten ; its pellicle is thinner and not so 
hard_ as that of tho wheat of the very cold table-lands of 
Mexico. All acre* near Victoria generally yields from 
three thousand to three thousand two hundred pounds 
weight of wheat. The average produce is consequently 
here, as at Buenos Ayres, three or four times as much as 
that of northern countries. Nearly sixteenfold of tho 
quantity of seed is reaped ; while, according to Lavoisier, 
* An arpent des eaux etfortts, or legi. acre of France, of wLch 1'95 
= 1 hectare. It is about 1J acre English. 
