Such phenomena as we have now described, and the causes of them which science is able to unfold, are worthy of consideration, in order to divest the mind of superstitious terrors, and enable it clearly to perceive the laws by which the Almighty directs the movements of the material system. When any appearance in nature, exactly the reverse of every thing we could have previously conceived--presents itself to view, and when we know of no material cause by which it could be produced, the mind must feel a certain degree of awe and terror, and will naturally resort to supernatural agency as acting either in opposition to the established laws of the universe, or beyond the range to which they are confined. Besides the fears and apprehensions to which such erroneous conceptions give rise, they tend to convey false and distorted impressions of the attributes of the Deity and of his moral government. Science, therefore, performs an invaluable service to man, by removing the cause of superstitious alarms, by investigating the laws and principles which operate in the physical system, and by assigning reasons for those occasional phenomena, which at first sight appeared beyond the range of the operation of natural causes. The late ingenious Dr. Wollaston illustrated the causes of some of the phenomena we have described, in the following manner. He looked along the side of a red hot poker at a word or object ten or twelve feet distant; and at a distance less than three eights of an inch from the line of the poker, an _inverted_ image was seen, and within and without that image, an _erect_ image, in consequence of the change produced, by the heat of the poker, in the density of the air. He also suggested the following experiment as another illustration of the same principle, namely, viewing an object through a stratum of spirit of wine lying above water, or a stratum of water laid above one of syrup. He poured into a _square_ phial a small quantity of clear syrup, and above this he poured an equal quantity of _water_ which gradually combined with the syrup, as seen at A. fig. 30. The word ‘Syrup,’ on a card held behind the bottle, appeared erect when seen through the pure spirit, but inverted, when seen through the mixture of water and syrup. He afterwards put nearly the same quantity of rectified spirits of wine above the water, as seen at B, and he saw the appearance as represented, namely, the true place of the word ‘Spirit,’ and the inverted and erect images below. These substances, by their gradual incorporation, produce refracting power, diminishing from the _spirit of wine_ to the _water_, or from the _syrup_ to the _water_; so that by looking through the mixed stratum, an inverted image of any object is seen behind the bottle. These experiments show that the _mirage_ and several other atmospherical phenomena may be produced by variations in the refractive power of different strata of the atmosphere. [Illustration: _figure 30._]