As yet, the reason why the dust falls, as it were, concretely, and not generally diffused through the atmosphere, is not known; it is one of the obscure points waiting further investigation. Why it should travel so far to fall in a particular spot is, in the present state of our knowledge, not easy to explain. The coarsest dust is generally the first to fall; and it seems clear, that the descent occurs when and where the conditions are favourable. Lieutenant Maury considers, 'that certain electrical conditions are necessary to a shower of dust as well as to a thunder-storm;' and that, in the periodical intervals, we may get a clue to the rate of motion of the upper aerial currents, which appear to be 'remarkable for their general regularity, their general direction, and sharpness of limits.' It is scarcely possible not to feel that the investigations here briefly sketched, possess unusual interest. As Ehrenberg says, the subject is one 'of vast, manifold, and rapidly-increasing importance, and is but the beginning of a future great department of knowledge.' Now that it has been published in a connected form, and the attention of scientific observers directed to it, we may hope soon to hear of corroborative evidence from all parts of the world. We may mention, as bearing on the question, that sand-showers are not unfrequent in China. Dr M'Gowan of Ningpo, in a communication to the Asiatic Society of Bengal, states, that at the beginning of 1851, three showers occurred within five weeks; the last, which commenced on the 26th March, and continued four days, being the heaviest. The wind during the time varied from north-east to north-west, the breeze interrupted by occasional calms. No rain had fallen for six weeks; and though, as the doctor observes, 'neither cloud, fog, nor mist obscured the heavens, yet the sun and moon were scarcely visible; the orb of day appeared as if viewed through a smoked glass, the whole sky presenting a uniform rusty hue. At times, this sameness was disturbed, exhibiting between the spectator and the sun the appearance of a water-spout, owing to the gyratory motions of the impalpable mineral. The sand penetrated the most secluded apartments; furniture wiped in the morning, would be so covered with it in the afternoon, that one could write on it legibly. In the streets, it was annoying--entering the eyes, nostrils, and mouth, and grating under the teeth. My ophthalmic patients generally suffered a relapse, and an unusual number of new cases soon after presented themselves. Were such heavy sand-storms of frequent occurrence, diseases of the visual organs would prevail to a destructive extent.'