Not a few inventors of the first mark are found among the men of great ability who unite training in two distinct fields of science, whose alliances they thoughtfully cultivate. New Modes of Attack. Thus Helmholtz, at once a physician and a physicist, devised the ophthalmoscope, that simple instrument for observing the interior of the eye. On a plane less lofty an inventor’s success may turn on his width of outlook, his intimacy with fields remote from the home acre, so that he may gainfully ally two arts or processes that, to a casual glance, seem utterly unrelated or unrelatable. When a pneumatic tube between a post-office and a railroad station is obstructed, there would seem to be no promise of aid in a fire-arm. But snapping off its blank cartridge at the open end of the tube gives back an echo through the air within the tube; in measuring the interval between touching the trigger and hearing the echo, there is news as to where the tube is choked, the velocity of sound in air being known. From the labors of a postmaster let us turn to those of an apothecary, who pounds and grinds his drugs in a mortar which has descended from the day when it reduced grain to flour. The grindstones which succeeded the mortar were only in recent years ousted by Hungarian rollers of steel which separate the constituents of grain with a new perfection. Their excellence consists in imitating the crushing of the mortar, not in attempting the grinding of the familiar burrs. The miller’s practice in one particular has given the postmaster a hint of value. In a flour-mill a cheap and sufficient motor is simple gravity as the products pass from one machine to the next. At the very outset the wheat is taken by conveyors to the top floor, whence its products descend, stage by stage, impelled by gravity alone, until the finished and barreled flour rolls into shipping rooms beside the railroad tracks. This principle has been adopted at the Chicago Post-office, where the mails as received are borne to the top floor, thence, by gravity, they take their way as sorted and re-sorted, to the ground floor where they are finally disposed of. In a field somewhat parallel is the modern art of designing the layout of a great manufacturing plant so that the material shall travel as little as possible between its entrance and its exit. In a well planned ship-yard the machines are so placed that the steel plates, bars and girders, the planks and boards, move continuously from one machine to its neighbor, ending at last by reaching the building berth.