168 
CLIMATE. 
Tables I. and II., observations daily at meridian. 
Table III., observations made twice and three times daily : chiefly at 10 
A. M. and 4 P. M. 
These curves in Tables I. and II., therefore, indicate the highest 
range of temperature on the northern coast, inasmuch as they 
are drawn from meridian observations, which give a higher 
average than those taken at 10 A. M. and 4 P. M. daily would 
yield ; and also because they are from the sea-level, which is 
always warmer than the upland. 
The thermal curve of December, 1850, and January, 1851, 
shows the influence which the fall of rain has upon' temperature, 
by producing a mean depression of about 12°. This low tem- 
perature prevails through the succeeding months, the warmth 
gradually augmenting till it attains the mean temperature of 
83° in April. 
The temperature of the sloping plains of the north is several 
degrees below that given on the tables, and is considerably less 
between the Jaltepec and Malatengo rivers, and somewhat less 
still between the latter river and the summit-level. 
The Jaltepec, where it crosses the line of survey, lies on a level of 75 ft. 
" Malatengo, 300 ft. 
u Summit-level, 800 ft. 
As the temperature diminishes on elevated land at the rate 
of one degree Fahrenheit for every 334 feet of elevation, it fol- 
lows that the mean temperatures of the Malatengo river-level 
and the summit-level would be always one and two degrees less 
respectively, throughout the year, than those given in the tables 
accompanying. But even this estimate is too high, for as other 
circumstances than elevation determine the mean temperature of 
any place, so in this instance the insular character and north- 
ern slope of the Gulf plains diminish the temperatures several 
degrees below that of the sea-level : this is clearly indicated by 
the character of the vegetation, the abundance of dicotyledonous 
trees, the appearance of the tree fern, of jpinus abies, and other 
Alpine plants, which point out a modified and insular tem- 
perature. The profusion of the Myrtle and Laurel families of 
plants is proof that the temperature is not tropical but sub- 
