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THE POPULAR SCIENCE MONTHLY.

is upward, downward, or horizontal, according to the position of the center of gravity at the moment of the experiment. M. Plateau has clearly demonstrated that the normal position of this point varies with each species.

One very small group, the Cetoniadæ, fly with the wing-cases down—an interesting fact, for in this instance they act upon the axis of suspension, and effect a step toward the state of complete differentiation which we find in the following group.

In the Diptera (mosquitoes, flies, etc.) the steering faculty reaches its highest development. The second pair of wings is transformed into organs having the special function of steering, the balancers or poisers; and these insects have accordingly a remarkable perfection of movement. A single pair of wings does all the flying, and, as they are not large, the diminution of the supporting surface is compensated by greater rapidity of vibration. I have proved by experiment that the balancers act by displacing the axis of suspension. Suppress the balancers, and the flight becomes fatally downward, because the normal and invariable position of the center of gravity is in front of the axis of suspension; the animal, therefore, can not modify his movement in any way, the abdomen being but slightly movable, and the balancers cut off. If, now, we come to his relief and attach a tiny weight to his abdomen, just sufficient to carry the center of gravity back to its normal place, we restore to the insect the power to perform all his aërial evolutions.

 

SKETCH OF JAMES CLERK MAXWELL.

AMONG the present generation of English physicists none have attained to greater eminence, or have made more valuable additions to this department of science, than the late Professor Maxwell. The splendid promise that his accomplished work gave of future work makes his death, at the early age of forty-nine, at the height of his powers, an irreparable loss to science. An accomplished mathematician, an unexcelled experimenter, he was peculiarly fitted to carry on those delicate researches in the domain of molecular physics by which he made it his own, and in which he was without a rival. Possessed of a vivid imagination, he had that power of holding it well under control, and making it subservient to the conditions of scientific investigation, that belongs only to the highest types of mind, and which is essential to the best and most valuable work in science. Though possessed of the power of direct and lucid exposition, he was never what is termed a popular lecturer. The subjects he considered, and his con-