ON UNDERGROUND TEMPERATURE. 



190 



uniform, and tho whole increase from this degth to the lowest reached is 

 3°-7, which is at tho rate of 1° F. for 68 feet. 



The elevation of Manegaon is estimated at L400 feet. It lies "in an open 

 valley of the Satpuras, traversed by the Dudhi river, south of tho wide plains 

 of the Narbada valley, about halfway between Jabalpur and Hoshungabad, 

 which arc 150 miles apart." Jabalpur is 1351 feet above sea-level, and has 

 a mean annual temperature of 75 0, 2. Hoshungabad is 1020 feet above sea- 

 level, and has a mean annual temperature of 78 3 vi. 



" The geological conditions of the position are favourable for these obser- 

 vations. The rocks consist of steady alternations, in about equal proportions, 

 of fine softish sandstones and hard silty clays of the Upper Gondwana strata, 

 having a steady dip of about 10°. . . . Strong trap-dykes are frequent in many 

 parts of the stratigraphical basin ; but there are none within a considerable 

 distance of these borings. There are no faults near, nor any rock features 

 having a known disturbing effect upon the heat-distribution."' 



Mention was made in last Report (p. 209) of two methods, which had 

 been suggested by members of the Committee, for plugging bores to prevent 

 the convection of heat. Mr. Lebour, at the request of the Committee, has con- 

 ducted experiments during the past year on both forms of plug. He reports 

 t hat — " In accordance with Sir W. Thomson's suggestion, disks of india-rubber 

 fixed to the lowering wire above and below the thermometer have been tried. 

 The chief difficulty met with was the unwieldiness of the armed portion of 

 the wire, which could not be wound and unwound from the drum, owing to 

 the fixed disk-holders. This difficulty prevented the placing of the disks any- 

 where but at the extremity of the wire, whereas it would be very desirable 

 to have a large number of them at intervals along the greater part of its en- 

 tire length. Disks for a 21-inch bore were found to work well with a dia- 

 meter of 2-1- inches. The lowering and especially the raising of the wire 

 armed with the disk-plugging were very slow operations, owing to the resist- 

 ance opposed by the water to the passage of the disks." 



Experiments with the form of plug devised by Mr. Lebour himself were 

 continued with a set of better made plugs. " The great disadvantage of this 

 system of plugging is the necessity of using two wires, one to lower the 

 thermometer and plug as usual, and the other to let down weights upon the 

 upper ends of the plugs when they are to be expanded, and to remove them 

 when they are to be collapsed. This necessitates not only the ordinary drum 

 for the first wire, but also an independent reel for the second. With care, 

 however, and after some practice, the apparatus was found to work well ; but 

 it certainly is extremely inconvenient for rapid work, as it requires a good 

 deal of setting up." 



Experiments were made with both forms of plug at the depth of 800 feet, 

 in a bore of the total depth of 420 feet. In the one case eight india-rubber 

 disks were employed, four above and four below the thermometer ; in tho 

 other, two collapsible plugs, one above and one below. 



The experiments had chiefly in view the mechanical difficulties of the sub- 

 ject, and are not decisive as to the sufficiency of the plugs to prevent convection. 



