The central belt of rains in midsummer over Africa, extends up as far as 17° north latitude, and perhaps further. North of this line over the whole surface of the desert, the Barbary States, a part of the Mediterranean, and some portion of Italy, the dry season extends, and from the entire surface the N. E. trade blow into the central belt.[1] Over the desert they all pass. Now this desert is a sea of sand, under a vertical sun, intensely heated, blistering the skin with which it comes in contact, and often acquiring a temperature of 150° to 160° of Fahrenheit. Under the central belt of rains neither the earth nor air exceed the temperature of 84°. And yet the hot air of the desert does not ascend, but blows into this cooler central belt; and when it is felt as it blows off the western coast by the mariner, or even in Guinea, when the belt of rains has gone south in winter, as it often is as the harmattan, it is suffocating and intolerable. There, then, not only is it untrue, that the land and the air over it under the rainy belt are hotter, but it is true that intensely heated air blows horizontally from the Desert of Sahara. Nay, as it will appear in the sequel, this hottest of all surfaces not only can not have a vortex, but it can not induce a monsoon, and scarcely a sea breeze. The same is true in a great degree of the surface, and the air over it, on either side of the supposed vortex of the rainy belt upon South America. See the description of Humboldt, already given, where the thermometer stood as high as 115° of Fahrenheit in the shade, while the N. E. winds, the regular trades, were blowing over the land. And it is equally true of Arabia, and indeed of every portion of the earth. There is not a spot upon the globe where the land and the air are cooler by the side of the central belt of rains, than under it. And the opposite is true every where upon the land.
How much hotter is the ocean and air under this supposed vortex? But little hotter than they are on the side where the sun is not vertical, and none on the other. Let us be a little more particular. The temperature of the Atlantic under the belt of rains in our winter, and on the south of the belt at the latitude of 3° south, and down to 9° or more south, is 82°. The air may range a degree, or possibly two, higher than the water at either point. On the north this difference is from nothing at the meeting of the trades and belt of rains, to about 4° at their northern limit. This is too trifling to be worth one moment’s consideration. It is less, far less than the difference between the water and air of the Gulf Stream which runs along our coast, and the adjoining waters and air over them. While on the south side of the belt of rains the difference is actually against the theory—and the same state of things is reversed in summer, when the sun is vertical at the north.
From the log of an intelligent shipmaster, found in the wind and current charts of Lieutenant Maury, I abridge the following, which will illustrate this. Captain Young in February, found the N. E. trades at about 17° north latitude, with the water at 75° and air at 76°, trade-wind N. E.
| At | 12° 16′ | the water was | 75° | the air | 76° | wind | N. E. | |
| Feb. | 22nd. | 9° 49′ | " | 76½° | " | 77° | " | N. E. |
| " | 23d. | 7° 13′ | " | 78° | " | 78° | " | N. E. |
| " | 24th. | no obs. | " | 79½° | " | 79° | " | N. E., E. S. E. rain. |
| " | 25th. | 3° 10′ | " | 81° | " | 83° | " | E. S. E. rain. |
| " | 26th. | no obs. | " | 82° | " | 82° | " | S. E. to E. S. E. hazy, rain & sqs. |
| " | 27th. | 2° 24′ | " | 82° | " | 82° | " | calm, with rain. |
| " | 28th. | no obs. | " | 82° | " | 82° | " | calm rain. |
| March | 1st. | 0° 29′ | " | 82° | " | 82° | " | E. S. E. sqs. rain. |
| " | 2nd. | 1° 27′ S. L. | " | 82° | " | 82° | " | S. E. sqs. rain. |
| " | 3d. | 2° 44′ | " | 82° | " | 83° | " | S. E. & S. S. E. weather settled. |
| " | 4th. | 4° 17′ | " | 82° | " | 83° | " | S. S. E. & S. E. fair weather. |
| " | 5th. | 6° 08′ | " | 82° | " | 84° | " | S. E. fair wthr. |
| " | 6th. | 8° 08′ | " | 82° | " | 84° | " | S. E. & E. S. E. fair weather. |
Here the air was seven degrees colder at the extreme limit of the N. E. trades than in the center of the belt of rains, as it is, usually, in mid-winter, but not in summer. On the other hand, after he left the region of calms and rains, where the water and air stood with almost entire uniformity at 82°, on the 3d of March, and for three days thereafter, during which he was in the S. E. trades with fair weather, the water was the same as under the supposed vortex, viz., 82°, and the air rose to 83° and 84°! This is demonstration.
I also take from a letter of Lieutenant Walsh to Lieutenant Maury, relative to the cruise of the “Taney” the following, showing the warmth of the Gulf Stream compared with the adjacent ocean.
“We first crossed the Gulf Stream on the 31st of October; we struck it in latitude 37° 22′, longitude 71° 26′ as indicated by the temperature of the water, which was as follows:
| 8 | A.M. | water at | surface | 66° |
| 9 | " | " | " | 73° |
| 10 | " | " | " | 76° |
| 11 | " | " | " | 77° |
77° was the highest temperature found in crossing at this time.
Re-crossing it in May, in latitude 35° 30′, longitude 72° 35′, he found the water as follows: