We have seen the fine old cathedral in this city, and the porcelain manufactory, both of which I had intended to describe to you; but my aunt recommends us to go to bed, as we are to be up very early to-morrow morning, in order that there may be full time for seeing the carpet manufactory at Kidderminster, on our way to Shrewsbury, where we are to sleep. So, good-night, though it is scarcely yet dark. What charming long days there are in this country compared with those of Rio.
14th June, Shrewsbury.
Sweet is the dubious bound
Of night and morn, when spray and plant are drenched
In dew.
Everything was in that state when we set out early this morning from Worcester; it reminded me of all my uncle had told me about dew, and I took the opportunity of asking him if dew is formed in the morning—“it continues to form in shaded places, after sun-rise,” said he; “but there is a shorter interval between sun-rise and its ceasing to form, than between its first appearance in the afternoon and sun-set; though Dr. Wells thinks, that if the weather be favourable, more dew forms a little before and a little after sun-rise, in shaded places, than at any other time.”
My aunt remarked, that a few years ago, while in constant attendance on a sick child from July to September, she rose every morning at day-break; and had an opportunity of observing, that about an hour before sun-rise the dew was particularly abundant. The window was frequently kept a little open at night, when the room was close, and the weather still; but the air became so chilly just as this heavy dew came on, that she was always obliged to shut it; yet during the night the chill was never perceived; which corroborates what Dr. Wells says, “that the cold of the atmosphere is greater in the latter than the prior part of the night.”
In the course of Dr. Wells’ observations, he found that dew does not form readily on gravel-walks; and that if the atmosphere be clear, neither the road or pavement are moistened with dew, though the grass on the road side, and painted doors and windows, are frequently wet. He found also, that wool, though highly attractive of dew, was prevented, if placed on a gravel-walk, from acquiring as much dew as an equal parcel of wool, if laid upon grass.
I asked why Dr. Wells used wool in these experiments, and my uncle told me, that at first he had only compared the quantities of dew on bodies having smooth surfaces; but that he found wool much better adapted to collect dew from the atmosphere, as it readily admits the moisture amongst its fibres, and retains what it receives very firmly. Filamentous and downy substances are by far the most productive of cold, such as wool, cotton, and flax, and still more fine raw silk and swan-down; all these were more steadily cold upon clear nights than even the grass; but swan-down showed the greatest cold.
“I have already explained to you,” continued my uncle, “that the surface of the earth, and all substances upon it, radiate back into the sky, at night, the heat which they receive in the day; and that, when this radiation is unobstructed by clouds, the cold it produces is proportionably greater. But the degree of cold is very much augmented when the form or situation of these substances prevents their deriving fresh supplies of heat from warmer bodies in contact with them, or in their neighbourhood. Most of the substances which I have named are not only naturally bad conductors of heat, but their form scarcely permits them to transmit from fibre to fibre any heat they might acquire. This is the reason why dew appears in greater quantity on shavings of wood, than on a thick piece of wood; and why filamentous substances become colder than all others.
“On a dewy evening the Doctor depressed a small tumbler into the soft garden mould, so that the brim was level with the ground; and he placed another standing on the surface of the mould: in the morning the former was dry in the inside, while that which stood on the surface was dewed; and the thermometer being applied to each, the heat of the depressed one was found to be 56°, while the other was only 49°; for not only had the upper glass more readily parted with its heat by radiation, but the other had received a constant supply of heat from the surrounding earth. In the same manner it may be explained, why the prominent parts of bodies are often encrusted with hoar frost, while the more solid and retired parts are free from it.”
I then inquired, why there is less dew of a windy evening; for one would suppose, that wind, instead of preventing the radiation of heat, would rather help to promote it.