( 36 )

preffes its a&ion in the direction of the plane;the fame laws apply to the vouffoirs of anarch. The objeCt of inquiry, in our endea-vours to obtain an arch of equilibration, isthe abfolute weight of each vouffoir in itsoblique direction; whether ariiing from thematerials folely of which the arch is com-pofed, or partly from the weight incumbent:and it is always to be kept in mind, thatwhen gravity is the foie aCting power in acompofition of lubricous forces, that an addi-tional force aCting vertically, and not deftroy-ing the equilibration, cannot alter the direc-tions of the parts; and, confequently, thatthe equilibration of an arch muff invariablydepend upon equable actions in the directionsof its vouffoirs. It is, perhaps, not unnecef-fary again to repeat, that the directions of thevouffoirs fhould be at right angles to the tan-gents of the refpeCtive parts of the intrados ofthe arch which they occupy.

In controverting the principles in the workabove alluded to, this opportunity is taken ofacknowledging the information which hasbeen derived from it; and it is hoped, that

other