Another fearful compensation for the mildness of the elements above the surface of the earth, is found in the frequency of subterranean disturbances. On an average, there are 45 shocks of earthquakes in the course of a year. These usually occur in the months from October to January, and again in May and June. But at intervals of from 40 to 60 years, the valley of the Rimac experiences an earthquake of far more desolating force, and by which Lima has several times been reduced to a heap of rubbish. The most destructive of these, since the European conquest were those of 1586, 1630, 1687, 1713, 1746, 1806--two in each completed century; so that the experience of the past gives us every reason to anticipate that many years will not elapse before Lima will once more become a mass of ruins. The most destructive of this regular series of great earthquakes was that of October 26, 1746. A little more than an hour before midnight, the earth began to tremble, and in three minutes from the time of the first shock, the city lay in ruins. Of more than 3000 houses, only 21 escaped entire. The towers of the Cathedral were overthrown. The bridge across the Rimac was almost the only public work which escaped, and of that one arch, upon which stood an equestrian statue of Philip V., was destroyed. But if Lima was sorely shattered, Callao was annihilated. The sea receded suddenly from the shore, and as suddenly rolled back with irresistible force, overwhelming the devoted city, with all its inhabitants, 5000 in number. Of these, it is popularly related, that only one escaped. A Spanish corvette which lay at anchor, was lifted sheer over the walls of the fortress, and deposited a full mile inshore, at a spot still designated by a cross erected to commemorate the fact. All the other vessels in the harbor were sunk. The modern town of Callao stands at the distance of two miles from the site of the old town, of which not a vestige remains. It is popularly affirmed that in a clear day the ruins of the old town may still be seen beneath the waves; but travelers, whose imagination is not keener than their vision, have vainly strained their sight to discover a trace of the lost city.