Prof. Henry communicated to the National Academy at Philadelphia his latest researches into the subject of sound, and among them an explanation of the echo observed on the water. This echo he had formerly been inclined to attribute to reflection from the crests of the waves. Tyndall holds that it is due to reflection from strata of air at different densities. Prof. Henry's present explanation is that this echo is produced by the reflection of the sound wave from the uniform surface of the water. The effect of the echo is produced by the fact that the original sound wave is interrupted. It has what the learned Professor calls shadows, produced by the intervention of some obstacle in its path. Sound is not propagated in parallel, but in diverging lines, and yet there are some cases where what may be called a "sound shadow" is produced. For instance, let a fog-signal be placed at or near water level on one side of an island that has a conical elevation. Then the signal will be heard distinctly by a vessel on the opposite side of the island at a distance of three miles. But when the vessel sails toward the island (the signal being on the opposite side), the sound will be entirely lost when the distance is reduced to a mile, and in any smaller distance it is not recovered. In this case the station of the vessel at the shorter distance is in the "sound shadow." The termination of that shadow is the point at which the diverging beams of sound, passing over the crest of the island, bend down and reach the surface of the water. The formation of the sound echo may be explained by this extreme divergence of the sound waves, for it is rational to suppose that at a great distance from the source of sound some of the dispersed waves will reach the water surface at such an angle as to be reflected back to the hearer. This was well illustrated by an experiment made to test Tyndall's theory. A steam siren was pointed straight upward to the zenith, but no echo from the zenith was heard, though the presence of a cloud from which a few raindrops fell certified the presence of air strata of different densities. But, strange to say, an echo was heard from every part of the horizon, half of which was land and half water. The only explanation of this fact is that the sound waves projected upward were so dispersed as to reach the earth's surface at a certain distance, and at that point some of them had curled over and assumed a direction that caused their reflection back to the siren.
THE DELICACY OF CHEMISTS' BALANCES.
In making chemical balances for fine work the beam is made in the truss form to prevent the bending which takes place even under such small loads as an ounce or two. Prof. Mendeleef has a balance that will turn with one-thousandth of a grain, when each pan is loaded with 15,000 grains. This extreme sensibility is obtained by the use of micrometer scales and cross threads at the end of the beam, these being observed by means of a telescope. Of course one weighing with this complicated apparatus occupies a long time. In most balances the beam rests on steel knife edges; but a maker who has lately obtained celebrity makes his supports of pure rock crystal. The steel edges can be seen with the naked eye; the quartz edges cannot be seen even with a magnifying glass. One writer on this subject thinks that with these perfect crystal edges, with an inflexible girder beam, a short beam giving quick vibrations, and a sensitiveness that can be increased by screwing up the centre of gravity, there can hardly be a practical limit to the smallness of the weight that will turn the beam. The amount of motion may be very small, but if this can be observed, the limit of possible accuracy is very much extended.
GOVERNMENT CONTROL OF THE DEAD.
What the population of European countries was a hundred years ago it would be hard to tell with accuracy; but the nations have doubled and trebled in strength within the century. Sanitary precautions have increased in importance, and the very noticeable movement in regard to social hygiene which now possesses English society is perhaps due in part to the obvious dangers to which thirty million human beings are subjected when living together on such a small area. The medical officer for Birkenhead has pointed out that it may be necessary for the government authorities to take more complete charge of the dead as a possible source of infection. He says that the intelligence of deaths from infectious diseases now furnished by local registrary would be much more useful than it is as a means for limiting the spread of disease if the medical officer were vested with further powers in respect to the infected dead body. At present neither the medical officer nor any one else has any power to order the immediate removal of an infected body, and those in charge of it might do what they liked with it. He advocated the necessity of power being given to medical officers to order the immediate removal of the infected bodies to a public mortuary and their speedy burial.
MICROSCOPIC LIFE.
Dr. Leidy lately described to the Academy of Sciences in Philadelphia an encounter for life which he witnessed between two microscopic animalcules. The two creatures were respectively 1-625th and 1-200th of an inch in diameter. On the morning of August 27, from some mud adhering to the roots of sphagnum, obtained the day previously in a nearly dried-up marsh at Bristol, Pennsylvania, he obtained a drop of material for examination with the microscope. After a few moments he observed an amoeba verrucosa, nearly motionless, empty of food, with a large central vesicle, and measuring 1/25th of a millimetre in diameter. Within a short distance of it, and moving directly toward it, was another and more active amoeba, regarding the species of which he was not positive. It was perhaps the one described by Dujardin as amoeba limax, by which name it may be called. As first noticed, this amoeba was one-eighth of a millimetre long, with a number of conical pseudopods projecting from the front border, which was one-sixteenth of a millimetre wide. The creature contained a number of spherical food spaces with sienna colored contents, a large diatom filled with endochrome, besides several clear food spaces, a posterior contractile vesicle, and the usual glanular endosarc. The amoeba limax approached and came into contact with the motionless amoeba verrucosa. Moving to the right, it left a long finger-like pseudopod curved around its lower half, and then extended a similar one around the upper half until it met the first pseudo-pod. After a few moments the ends of the two projections actually became continuous, and the verrucosa was enclosed in the embrace of the amoeba limax. The latter assumed a perfectly circular outline, and after a while a uniformly smooth surface. It now moved away with its new capture, and after a short time what had been the head end contracted and became wrinkled and villous in appearance, while from what had been the tail end ten conical pseudopods projected. The amoeba verrucosa assumed an oval form, and the contractile vesicle became indistinct without collapsing. Moving on, the amoeba limax became more slug-like in shape. The amoeba verrucosa now appeared enclosed in a large oval, clear vacuole or space, was constricted so as to be gourd-shaped, and had lost all trace of its vesicle. Subsequently it was doubled upon itself, and at this point the amoeba limax discharged from one side of the tail end the siliceous case of the diatom, which now contained only a shrivelled cord of endochrome. Later the amoeba verrucosa was broken up into fine spherical granular balls, and these gradually became obscured and apparently diffused among the granular contents of the endosarc of the amoeba limax. The observations from the time of the seizure of the amoeba verrucosa to its digestion or disappearance among the granular matter of the entosarc of its captor, occupied seven hours. From naked amoeba the shell-protected rhizopods were no doubt evolved, and it is a curious sight to observe them swallowed, home and all, to be digested out of their house. It was also interesting to observe the cannibal amoeba swallowing one of its own kind and appropriating its structure to its own use, just as we might do the contents of an egg. The amoeba verrucosa he describes as remarkable for its sluggish character, and in appearance reminds one of a little pile of epithelial scales or a fragment of dandruff from the head. It is oval or rounded, transparent, and more or less wrinkled, or marked with delicate, wavy lines.