GYROSCOPE (jir'6), an instrument constructed by M. Foucault, to make the rotation of the earth visible. The prin ciple on which it proceeds is this—that, unless gravity intervenue, a rotating body will not alter the direction in which its permanent axis points. In the gyro scope there is a rotating .metallic disk, the middle point of whose axis is also the center of gravity of the machine. By this device the action of gravity is eliminated. The instrument, moreover, is so con structed that the axis of rotation can be made to point to some star in the sky. Then, as the heavy disk whirls round, it is found that the axis continues to point to the moving star, though, in conse quence of this, apparently altering its direction relatively to bodies on the earth. If, again, the axis be pointed to the celestial pole, which is fixed, no alteration in its position relative to bodies on the earth takes place. The only fea sible explanation of these appearances is that the earth is revolving on its axis. The gyroscope has been a valuable ad junct as a stabilizer in the making of aeroplanes.
H, h, the eighth letter of the English alphabet, commonly classed among the consonants, though not strictly such. It is frequently called the aspirate, though other letters are also aspirated in Eng lish. Its distinctive or proper sound is
that which it has at the beginning of a word, as help, hard, hope, etc. This sound it also has when following w, as in whither, where, in which cases it originally preceded the w, as in Old Eng lish, hwidir, hwter. H has disappeared from many words, especially before 1, n, r: as, Old English, hlaf=loaf; Old Eng lish hnecca=neck; Old English hring= ring; Old English feoh=fee. It has been intruded into some words as wharf, whelk, whelm. H is commonly joined to other consonants to form digraphs repre senting sounds for which there are no symbols in the alphabet; as, ch in child, chill; sh in shin, ship; th in this, that, thine; joined with p it forms the sound of f; with g it sometimes forms the sound of f, as in enough, tough; sometimes the digraph is silent, as in bough, plough. The combination rh is found only in words derived from the Greek, where the h represents the rough breathing of the original rh. Ch is common in words derived from the Greek, and in such cases is generally hard, as chemistry, chyle, etc. It sometimes represents the Latin c, Greek k, as in English horn=Lat. cornea, Gr. keras.