[433] Balloon ascents and observations on small mountainous islands, therefore, offer the best means of solving such questions: of these, the results of ballooning, under Mr. Welsh’s intrepid and skilful pioneering (see Phil. Trans. for 1853), have proved most satisfactory; though, from the time for observation being short, and from the interference of belts of vapour, some anomalies have not been eliminated. Islands again are still more exposed to local influences, which may be easily eliminated in a long series of observations. I think that were two islands, as different in their physical characters as St. Helena and Ascension, selected for comparative observations, at various elevations, the laws that regulate the distribution of humidity in the upper regions might be deduced without difficulty. They are advantageous sites, from differing remarkably in their humidity. Owing partly to the indestructible nature of its component rock (a glassy basalt), the lower parts of Ascension have never yielded to the corroding effects of the moist sea air which surrounds it; which has decomposed the upper part into a deep bed of clay. Hence Ascension does not support a native tree, or even shrub, two feet high. St. Helena, on the other hand, which can hardly be considered more favourably situated for humidity, was clothed with a redundant vegetation when discovered, and trees and tree-ferns (types of humidity) still spread over its loftiest summits. Here the humidity, vegetation, and mineral and mechanical composition reciprocate their influences.
The great amount of relative humidity registered at 6000 to 8000 feet, arises from most of the observations having been made on the outer range, where the atmosphere is surcharged. The majority of those at 10,000 to 12,000 feet, which also give a disproportionate amount of humidity, were registered at the Zemu and Thlonok rivers, where the narrowness of the valleys, the proximity of great snowy peaks, and the rank luxuriance of the vegetation, all favour a humid atmosphere.
I would have added the relative rain-fall to the above, but this is so very local a phenomenon, and my observations were so repeatedly deranged by having to camp in forests, and by local obstacles of all kinds, that I have suppressed them; their general results I have given in Appendix F.
I here add a few observations, taken on the plains at the foot of the Sikkim Himalaya during the spring months.
Comparison between Temperature and Humidity of the Sikkim Terai and Calcutta, in March and April, 1849.
| No. of Obs. | Locality | Elev. above sea. Feet | TEMPERATURE | DEW POINT | TENSION | SAT. | ||||
| C. | T. | C. | T. | C. | T. | C. | T. | |||
| 4 4 3 3 4 3 2 8 | Rummai Belakoba Rangamally Bhojepore Thakyagunj Bhatgong Sahibgunj Titalya | 293 368 275 404 284 225 231 362 | 82·2 92·8 84·2 90·1 84·9 87·4 80·2 85·5 | 70·6 85·5 75·0 81·2 77·1 74·9 68·0 80·0 | 61·7 62·6 68·7 54·1 61·3 64·7 66·2 55·4 | 60·5 63·0 62·5 44·3 60·8 54·6 53·1 56·1 | ·553 ·570 ·695 ·429 ·547 ·611 ·642 ·448 | ·532 ·578 ·568 ·308 ·537 ·436 ·414 ·459 | ·517 ·382 ·605 ·313 ·466 ·480 ·635 ·376 | ·717 ·485 ·665 ·295 ·588 ·512 ·409 ·459 |
| 31 | Means | 305 | 85·9 | 79·0 | 61·8 | 56·9 | ·562 | ·479 | ·472 | ·516 |
| May, 1850 Kishengunj | 131 | 89·7 | K 78·6 | 76·7 | K 71·4 | ·904 | K ·759 | ·665 | K ·793 | |
| Vapour in a cubic foot | |||
| Kishengunj Calcutta | 8·20 9·52 | Terai Calcutta | 5·08 5·90 |
| Mean difference of temperature
between Terai and Calcutta, from 31 observations in March, as above, excluding mimima, Terai Mean difference from 26 observations in March, including mimima, Terai Mean difference of temperature at Siligoree on May 1, 1850 Mean difference of temperature at Kishengunj on May 1, 1850 | 6·9 9·7 10·9 11·1 |
From the above, it appears that during the spring months, and before the rains commence, the belt of sandy and grassy land along the Himalaya, though only 3·5° north of Calcutta, is at least 6° or 7° colder, and always more humid relatively, though there is absolutely less moisture suspended in the air. After the rains commence; I believe that this is in a great measure inverted, the plains becoming excessively heated, and the temperature being higher than at Calcutta. This indeed follows from the well known fact that the summer heat increases greatly in advancing north-west from the Bay of Bengal to the trans-Sutledge regions; it is admirably expressed in the maps of Dove’s great work “On the Distribution of Heat on the Surface of the Globe.”
H.
ON THE TEMPERATURE OF THE SOIL AT VARIOUS ELEVATIONS.