There are numerous familiar effects which are manifestations of the principle now explained. When a vessel of milk is allowed to remain a certain time at rest, it is observed that a stratum of fluid will collect at the surface, differing in many qualities from that upon which it rests. This is called _cream_; and the property by which it ascends to the surface is its relative levity; it is composed of the lightest particles of the milk, which are in the first instance mixed generally in the fluid; but which, when the liquid is allowed to rest, gradually arise through it, and settle at the surface. _Directions engraved upon the Common Weather Glasses absurd._ The barometer has been called a _weather glass_. Rules are attempted to be established, by which, from the height of the mercury, the coming state of the weather may be predicted, and we accordingly find the words "Rain," "Fair," "Changeable," "Frost," &c., engraved on the scale attached to common domestic barometers, as if, when the mercury stands at the height marked by these words, the weather is always subject to the vicissitudes expressed by them. These marks are, however, entitled to no attention; and it is only surprising to find their use continued in the present times, when knowledge is so widely diffused. They are, in fact, to be ranked scarcely above the _vox stellarum_, or astrological almanac. Two barometers, one near the level of the River Thames, and the other on the heights of Hampstead, will differ by half an inch; the latter being always half an inch lower than the former. If the words, therefore, engraved upon the plates are to be relied on, similar changes of weather could never happen at these two situations. But what is even more absurd, such a scale would inform us that the weather at the foot of a high building, such as St. Paul's, must always be different from the weather at the top of it. It is observed that the changes of weather are indicated, not by the actual height of the mercury, but by its _change_ of height. One of the most general, though not absolutely invariable, rules is, that when the mercury is very low, and therefore the atmosphere very light, high winds and storms may be expected. The following rules may generally be relied upon, at least to a certain extent: 1. _Generally_ the rising of the mercury indicates the approach of fair weather; the falling of it shows the approach of foul weather. 2. In sultry weather the fall of the mercury indicates coming thunder. In winter, the rise of the mercury indicates frost. In frost, its fall indicates thaw; and its rise indicates snow. 3. Whatever change of weather suddenly follows a change in the barometer, may be expected to last but a short time. Thus, if fair weather follow immediately the rise of the mercury, there will be very little of it; and, in the same way if foul weather follow the fall of the mercury, it will last but a short time.