The road being highly cultivated, and the Date-palm becoming more abundant, we encamped in a grove of these trees. All were curiously distorted; the trunks growing zigzag, from the practice of yearly tapping the alternate sides for toddy. The incision is just below the crown, and slopes upwards and inwards: a vessel is hung below the wound, and the juice conducted into it by a little piece of bamboo. This operation spoils the fruit, which, though eaten, is small, and much inferior to the African date.
At Mudunpore (alt. 440 feet) a thermometer, sunk 3 feet 4 inches in the soil, maintained a constant temperature of 71·5°, that of the air varying from 77·5°, at 3 p.m., to 62 at daylight the following morning; when we moved on to Nourunga (alt. 340 feet), where I bored to 3 feet 8 inches with a heavy iron jumper through an alluvium of such excessive tenacity, that eight natives were employed for four hours in the operation. In both this and another hole, 4 feet 8 inches, the temperature was 72° at 10 p.m.; and on the following morning 71·5° in the deepest hole, and 70° in the shallower: that of the external air varied from 71° at 3 p.m., to 57° at daylight on the following morning. At the latter time I took the temperature of the earth near the surface, which showed,
| Surface | 53° |
| 1 inch | 57° |
| 2 inches | 58° |
| 4 inches | 62° |
| 7 inches | 64° |
The following day we marched to Baroon (alt. 345 feet) on the alluvial banks of the Soane, crossing a deep stream by a pretty suspension bridge, of which the piers were visible two miles off, so level is the road. The Soane is here three miles wide, its nearly dry bed being a desert of sand, resembling a vast arm of the sea when the tide is out: the banks are very barren, with no trees near, and but very few in the distance. The houses were scarcely visible on the opposite side, behind which the Kymore mountains rise. The Soane is a classical river, being now satisfactorily identified with the Eranoboas of the ancients.[[19]]
[19] The etymology of Eranoboas is undoubtedly Hierrinia Vahu (Sanskrit), the golden-armed. Sons is also the Sanskrit for gold. The stream is celebrated for its agates (Soane pebbles), which are common, but gold is not now obtained from it.
The alluvium is here cut into a cliff, ten or twelve feet above the bed of the river, and against it the sand is blown in naked dunes. At 2 p.m., the surface-sand was heated to 110° where sheltered from the wind, and 104° in the open bed of the river. To compare the rapidity and depth to which the heat is communicated by pure sand, and by the tough alluvium, I took the temperature at some inches depth in both. That the alluvium absorbs the heat better, and retains it longer, would appear from the following, the only observations I could make, owing to the tenacity of the soil.
| 2 p.m. | Surface | 104° |
| 2·5 inches | 93° | |
| 5 inches | 88° | |
| Sand at this depth | 78° | |
| 5 a.m. | Surface | 51° |
| 28 inches | 68·5° |
Finding the fresh milky juice of Calotropis to be only 72°, I was curious to ascertain at what depth this temperature was to be obtained in the sand of the river-bed, where the plant grew.
| Surface | 104·5° | |
| 1 inch | 102° | |
| 2 inches | 94° | |
| 2·5 inches | 90° | |
| 3·5 inches | 85° | (Compact) |
| 8 inches | 73° | (Wet) |
| 15 inches | 72° | (Wet) |
The power this plant exercises of maintaining a low temperature of 72°, though the main portion which is subterraneous is surrounded by a soil heated to between 90° and 104°, is very remarkable, and no doubt proximately due to the rapidity of evaporation from the foliage, and consequent activity in the circulation. Its exposed leaves maintained a temperature of 80°, nearly 25° cooler than the similarly exposed sand and alluvium. On the same night the leaves were cooled down to 54°, when the sand had cooled to 51°. Before daylight the following morning the sand had cooled to 43°, and the leaves of the Calotropis to 45·5°. I omitted to observe the temperature of the sap at the latter time; but the sand at the same depth (15 inches) as that at which its temperature and that of the plant agreed at mid-day, was 68°. And assuming this to be the heat of the plant, we find that the leaves are heated by solar radiation during the day 8°, and cooled by nocturnal radiation, 22·5°.