TRANSACTIONS OP THE SECTIONS. 105 



proportionally and gradually increased, until it has traversed to the extent of its limit : 

 and as, during its passage, it will be gradually approximating to the upper sight, there 

 will be an increasing ratio of range as the distance between the two points of sight 

 diminishes. 



The plate upon which the lower sight traverses should be graduated throughout its 

 length for every ten yards of distance within the range of the barrel (the charge and 

 quality of the powder being uniform). By means of the thumb, the sight may be in- 

 stantly adjusted to accord with the estimated distance of the object from the observer. 



On the Granite Quarries of Dartmoorf-and their Railways and Machinery, 



By W. Johnson. 



The surface granite of Dartmoor, existing in detached blocks, has been long em- 

 ployed in the neighbourhood for ordinary building purposes, but the quarried granite 

 was first brought into the market by the Haytor Granite Company about the year 

 1820. The construction of a stone tramway allowed of the granite being shipped at 

 , Teignmouth ; it now competes with the best Aberdeenshire stone, since the lightness 

 of its tint, the fineness of its texture, and the very large blocks in which it can be ob- 

 tained, render it for some purposes unrivalled, and it has been extensively employed 

 in many public buildings, both in the metropolis and other places. The completion 

 in 1825 of the Plymouth and Dartmoor Railway, of the length of 25 miles and uni- 

 form rise of 1 in 94, aifords ready transport for the granite of the western face of the 

 moor from Fogginton and other parts adjacent, and the facilities with which these 

 quarries are worked are very great. Strong timber stages with travelling frames, and 

 upon the frames powerful traversing crabs, avoiding thereby the labour and delay of 

 lifting by the ordinary' means of derricks and cranes, are now in the course of con- 

 struction. The travelling frames, with the crabs upon them, can be transferred from 

 one fine of scafibld to another, so that power may be accumulated to any extent upon 

 one stage, so as to operate on blocks of extraordinary size. The magnitude of the 

 blocks in which the granite can be procured from this quarry, renders it peculiarly 

 fitted for the largest works of the engineer. The beds already accessible lie at great 

 depths below the surface, and yield stone of the greatest compactness, strength, and 

 uniformity of colour, and the horizontal disposition of the rock allows of the removal 

 of stone of fair forms and in blocks of the largest size. 



On Arnott's Stove, and the Construction of Descending Flues, and their Ap- 

 plication to the purposes of Ventilatioji. By J. N. Hearder. 



The general advantages of Arnott's stoves in economizing fuel, avoiding smoke, and 

 regulating the temperature, are well known ; but these stoves are attended with some 

 disadvantages, of which the danger of explosion and imperfect ventilation are the 

 most serious. The liability to explosion Mr. Hearder considers to arise from the 

 construction of the stove, in having the upper door air-tight, the only aperture for air 

 being the valve aperture of the ash-pit. The air so admitted is immediately decom- 

 posed, and nearly the whole of its oxygen is abstracted, so that by the time it has 

 passed up through the fuel, and reached the upper chamber of the stove, it has not oxy- 

 gen enough left to support combustion. The heat evolved by the lower stratum of fuel, 

 acting upon the upper stratum of fresh or unignited fuel, liberates from it the inflam- 

 mable gas which it contains, and which also accumulates in the top of the stove. A 

 mixture is then formed analogous to the fire-damp of coal mines, ready for explosion 

 whenever the requisite oxygen or degree of temperature shall be present. Under these 

 circumstances, should the door be opened, a burst of flame outwards may be the re- 

 sult ; or should a pufl:' of wind down the chimney carry the mixture down through 

 the ignited fuel, an explosion may ensue. Other causes, such as the sudden shutting or 

 opening of the door of an apartment, may occasion the downward draught and conse- 

 quent explosion. Now carburetted hydrogen will not explode when the proportion of 

 the air to the hydrogen exceeds a certain limit, so that if air be supplied to the top of 

 the stove, so as greatly to preponderate over the hydrogen, the latter will burn off in 

 flame at the moment of its formation, or be carried up the flue. Mr. Hearder, there- 

 fore, proposes as a remedy, perforations through the lower edge of the upper door, so 

 that air may be admitted on a level with the top edge of the fire-brick, through which 



