218 APPENDIX VII P \ the Edgar Thomson Works (American Iron & Steel Institute meet- ing, May 28, 1915), which showed that the checker openings close to the shell took more gas when heating and less air on blast. In his work, Fours d Flamme, Professor Groume-Grjimailo gives a mathematical demonstration of the reason why the portion of the checkerwork close to the wall, losing a considerable amount of heat by radiation, exerts a greater cooling effect on the hot gases of combustion flowing downward and tends to concentrate their flow through this portion of the checker, while the air, which is being heated, tends to seek the central and for that reason more slowly cooled portion of the checkerwork. It is interesting to note that Professor Groume-Grjimailo cites the Cowper stove, as follows: " For example, the checker openings of the Cowper hot-blast stove located nearest to the shell lose a great deal of heat by radiation; they therefore exercise a stronger cooling effect upon the current of gas flowing through them and by reason of this the velocity of the descending current of gas is increased, since if fe<^i then V2>vi." Convection currents in gases and air, due to small temperature differences, are by no means inconsiderable. A temperature dif- ference of 1° C. is sufficient to impress a vertical velocity of 0.268 m (0.88 ft) per second on a gas. This velocity will be increased directly as the square root of the difference in tempera- ture. Convection currents will act to carry the cooled gas or the heated gas or air away from the heating or cooling surface. These currents will exist in any chamber, regardless of whether a current of gas is circulating through the chamber or not. When a hot gas is giving up its heat to a surface the convection currents will be downward, while when a cool gas is absorbing heat from a surface the convection currents will be upward. It naturally follows that the working current or circulation of the air or gases should be in the same direction as the convection currents, as in this case they wi]l tend to reduce the friction loss. When the convection cur- rents are in the opposite direction to the working current they form recirculating loops which entail a direct loss of heat capacity in the checkerwork.