z 3 2 POPULAR SCIENCE MONTHLY. 



if you live on the equator — I use ordinary terms instead of astro- 

 nomical ones for simplicity's sake — -is so comparatively small that 

 within the tropics themselves you never notice much difference as 

 to the amount of heat between one period of the year and another. 

 In equatorial countries the day and night temperature is much the 

 same all the year round : if the country be plain, it is always hot ; if 

 mountainous, like the district about Bogota, it is " a perpetual 

 spring"; one day is always much the same as the one that went 

 before and the one that comes after it. Even on the actual tropics, 

 again, the difference is too slight to make any marked change in the 

 temperature; people living on the northern tropic (Cancer), for ex- 

 ample, have the sun vertical to them on June 21st, and some 

 forty-three degrees south of them on December 21st. Neverthe- 

 less, the sun is still as near them and as powerful as he is at Milan 

 or Venice in the height of summer; and the consequence is that, 

 as a matter of fact, the thermometer within the tropics and at sea 

 level seldom descends below 75° or 80°, even at midnight in the 

 relative winters. For the heating power of the sun depends, 

 of course, upon the directness of his rays, and lessens with their 

 obliquity; in Venice and Milan they are strong enough to make the 

 ground very hot in July and August, though it has been cooled be- 

 fore bv a northern winter; much more than in Jamaica or Mada- 

 gascar, which have never been cooled, does the accumulated heat 

 keep everything warm even when the sun is most oblique — and he 

 never reaches the same obliquity as in an English summer. The 

 ground is hot, the houses are hot, wood and stone are hot, and they 

 have all been hot from time immemorial. 



Yet tropical and equatorial trees and plants have their definite 

 seasons to flower and fruit, just the same as elsewhere. This seems 

 surprising at first when one visits the tropics. You can not see why 

 everything should not flower and fruit the whole year round. And 

 yet, at one time pineapples are " in," at another mangoes. And 

 these seasons differ in the northern and southern hemispheres; what 

 is mango winter in the one being mango summer in the other. I 

 do not say the seasons anywhere in the tropics differ markedly; still, 

 they do differ; the tropical year is divided into times and months 

 for agriculture just as much as any other. Thus there are regular 

 dates in each hemisphere for planting, tending, and cutting the 

 sugar cane. Now, what is the reason of these changes in vegetation, 

 when temperature remains so constant? Why do not trees and 

 shrubs of each kind flower up and down throughout the year irregu- 

 larly — now one individual and now another? Why are there seasons 

 for things at all in the tropics? 



The answer is, because the same causes which produce summer 



