An error of one foot in a thousand in determining the velocity of sound will affect the third decimal figure one or two units. A small difference in the assumed weight of a cubic foot of air also affects the result. M. Hanssen gives 0.080743 pound as the weight at 32 deg. F. under the pressure of one atmosphere; while Rankine gives 0.080728 pound. In my own computations I use 1.406 as a more probable value of the constant sought. This will give for the specific heat of air at constant pressure 0.2375 / 1.406 = 0.1689 This is only 0.0003 of a unit greater than the value used by Hanssen, but it would have given him nearly 775, instead of 771.89. Again, he uses 491.4 deg. F. for the absolute temperature of melting ice. The exact value of this constant is unknown; but the mean value as determined by Joule and Thomson, in their celebrated experiments with porous plugs, was 492.66 deg. F. This value would slightly change his result. It will be seen from the above that a small change in the constants used may affect by several units the computed value of the mechanical equivalent. I have computed it, using 1.406 for the ratio of the specific heat of air at constant pressure to that at constant volume, 491.13 deg. F. as the temperature of melting ice above the zero of the _air_ thermometer, 26,214 feet for the height of a homogeneous atmosphere, and 0.2375 for the specific heat of air, and I find, by means of these constants, 778. If computed from the zero of the absolute scale, 492.66 deg. F., I find 777 to the nearest integer. Recently I have used 778. If the value given by Rowland, about 783 according to the air thermometer at 39 deg. F., should prove to be correct, it seems probable that the constant 1.406 used above would be reduced to about 1.403, or that the other constants must be changed by a small amount. The height of the homogeneous atmosphere used above, 26,214 feet, is the value used by Rankine as deduced from Regnault's figures, and only one foot less than the value used by Sir William Thomson; but the figures used by Mr. Hanssen give 26,2101/2 feet. The method above called Hanssen's is really that of Dr. Mayer (the German professor), who in 1842 used it for determining the mechanical equivalent; but on account of erroneous data, the value found by him was much too small. * * * * * ECONOMY TRIALS OF A NON-CONDENSING STEAM ENGINE--SIMPLE, COMPOUND, AND TRIPLE.[1] [Footnote 1: Abstract of paper read before the Institution of Civil Engineers, March 13.] BY MR. P. W. WILLANS, M.I.C.E.