363 On the Position of Frames in a Sloping Gallery. [Oct, 



TO THE EDITOR OF THE MADRAS LITERARY JOURNAL. 

 SIR, 



You invite remarks on the subject of the position of frames in a 

 sloping gallery. I beg to offer the following. 



The first thing to be considered is what the frame is required to sup- 

 port : it can scarcely be said to be the whole mass above the excavation, 

 for all soils have more or less support derived from their own adhesion, 

 or the counterpressure of the neighbouring parts. Some soils require 

 no support, others (such as those from which only massive fragments 

 can detach themselves) may be supported by simple props, and these 

 must be placed vertically, unless the fragment can, by an oblique prop, 

 be brought to bear a portion of its weight upon a more solid body j 

 but I suppose here, soil of uncertain nature, and every fragment calling 

 for distinct support, to show 7 an instance in which vertical props must 

 be used. I will now T proceed to consider the case to which your corres- 

 pondents appear to refer, namely, where the soil is of such a nature as 

 to require frames and sheeting planks — now, what is the force exerted 

 which the frames have to resist ? I should say, in all cases, falling frag- 

 ments detached by the force of gravity from the mass above, variable 

 according to the nature of the soil, but exerting a vertical force. In 

 ordinary soil these fragments fall on the sheeting, striking a blow which 

 in some measure acts obliquely on the head of the frames if vertical, 

 on the principle that a plane, struck by a force in any direction, is only 

 affected in a direction perpendicular to kself. But, to fulfil this principle 

 the blow must be such as is struck by a ball, which glances; for, if the 

 ball penetrates, the force communicated to the plane, is in the direction 

 of the stroke. If the soil is moist and adhesive, it at once unites with 

 the sheeting, and acts only with a vertical force on the supports ; if 

 loose and dry, it falls on the plane and slides down, in some degree 

 acting as a stroke, but its effect will be somewhere between that of 

 the ball which penetrates and the ball which glances, between a verti- 

 cal direction and a direction perpendicular to the plane. If the parti- 

 cles are light, the effect in the latter direction must be wholly insignifi- 

 cant, yet this is the only way in which I can account for the tendency 

 to fall backwards, which vertical frames in a sloping gallery are found 

 to have ; for, w 7 hen the fragments cease to fall, and become settled on 

 the sheeting, they form one mass acting with the direct force of gravi- 

 ty. Unless we take altogether another view of the subject, and show that 

 practically a vertical support, to a sloping surface, is likely to slip from 

 under it and not act at all ; but this has nothing to do with the principle, 

 for if it acts at all ; that is, if there is friction enough to prevent its slip- 

 ping, it acts the same as if a notch or step were cut for it, and directly 

 opposes the vertical pressure. It would, therefore, appear that a trifling 

 obliquity is necessary, when the frames are first set up, but this neces- 



