h being the difference of level between the two ends of the portion of conduit of length, L, and the sign + or - being used according as the conduit rises or falls. The specific weight, [delta], is constant, and the quotients, p1/[delta] and p/[delta], represent the heights, z and z1, to which the water could rise above the pipes, in vertical tubes branching from it, at the beginning and end of the transit. The values assigned to the coefficient b1 in France, are those determined by D'Arcy. For new cast-iron pipes he gives b1 - 0.0002535 + 1/D 0.000000647; and recommends that this value should be doubled, to allow for the rust and incrustation which more or less form inside the pipes during use. The determination of this coefficient has been made from experiments where the pressure has not exceeded four atmospheres; within these limits the value of the coefficient, as is generally admitted, is independent of the pressure. The experiments made by M. Barret, on the pressure pipes of the accumulator at the Marseilles docks, seem to indicate that the loss of pressure would be greater for high pressures, everything else being equal. This pipe, having a diameter of 0.127 m. (5 in.), was subjected to an initial pressure of 52 atmospheres. The author gives below the results obtained for a straight length 320 m. (1050 ft) long; and has placed beside them the results which D'Arcy's formula would give. Loss of head, in meters or ft. respectively per 100 meters or ft. run of pipes. +-----------------^-------------------+ | | Calculated loss. +-----------^-----------+ | | Velocity of flow Actual loss per second. observed. Old pipes. New pipes. Meters. Feet. Met. or Ft. Met. or Ft. Met. or Ft. 0.25 0.82 1.5 0.12 0.06 0.50 1.64 2.5 0.48 0.24 0.75 2.46 3.7 1.08 0.54 1.00 3.28 5.5 1.92 0.96 1.25 4.10 6.1 3.00 1.50 1.50 4.92 7.3 4.32 2.16 1.75 5.74 8.0 5.88 2.94 2.00 6.56 10.2 7.68 3.84 2.25 7.38 11.7 9.72 4.86 2.50 8.20 14.0 12.00 6.00