7th.—My curiosity about frost has been gratified. Each of the last three nights the thermometer has been below the freezing point—last night it was 28°. The ground is hard, and grass, trees, and shrubs, are quite white. Nothing can be more beautiful—each blade of grass sparkling with gems, every branch and spray covered with delicate crystals, and the leaves of the fir-trees hanging like little miniature icicles.
I asked my uncle where the frost comes from. “It is in fact,” said he, “frozen dew; when the ground is cooled down to 32°, the dew deposited on it is congealed, and becomes hoar frost. This often happens when the temperature of the atmosphere is much higher; and I have seen a copious hoar frost in a clear calm night, though the air was not colder than 40°.”
When I begged my uncle to explain that, he told me that, from the satisfactory observations of Dr. Wells, it appears that the heat which the earth receives from the sun in the day is returned or radiated back again from the earth during the night, and is dispersed in the sky; the surface of the earth thus becomes cold from its sudden loss of heat, and congeals the dew. The cold produced by this radiation of heat from the earth, is always less if any substance be interposed between it and the sky; not only a solid body, but even a fog, or clouds, have this effect, because they intercept the heat, and perhaps again send back a portion of it to the earth; and this, he added, is the reason why a bright clear night is generally colder than a cloudy night.
I asked my uncle if that was also the reason that such slight substances, as straw or mats, are found to protect tender plants from cold?
“Yes,” said he; “I used to wonder how such thin, open things as Russia mats could prevent plants from becoming of the same temperature as the atmosphere; but when I learned that all bodies at night give out their heat by radiating it, unless some covering be interposed, which acts, not by keeping out the cold, but by preventing their heat from flying off, then I perceived the reason of what before had appeared to me to be almost useless.”
He described several experiments he had tried to satisfy himself on this subject. He found that even a cambric handkerchief was sufficient; and that when raised a few inches in the air, the warmth of the grass beneath was 3° greater than that of a neighbouring piece of grass which was sheltered by a similar handkerchief actually in contact with it. All his experiments confirmed those of Dr. Wells, and shewed that by placing substances for the shelter of plants, not directly touching them, the effect was increased. Snow acts in the same manner as a preservative of plants when the ground is not already frozen.
Some other experiments my uncle then described, and he endeavoured to make me understand Dr. Wells’s general opinions on the formation of dew. He also mentioned the curious method they have in India of forming artificial ice in earthen-ware pans, where the temperature of the air is even 12 or 14 degrees above the freezing point. He concluded by saying, “I do not tell you all these particulars, Bertha, merely to stuff your memory with philosophical shreds and patches, but to excite your mind to observation and inquiry, which is a hundred times more useful.”
8th, Sunday.—The Ephod being mentioned in a part of the Scripture I was reading this morning, I asked my uncle to describe it, for I had but a confused idea of the dress of the high priest. He says the name is derived from a Hebrew word, signifying to tie. It was made of linen, and brought from behind the back, over each shoulder; and then crossing the breast, it was passed round the waist so as to form a girdle; the two ends hanging down before. The Breast-plate of Judgement, which was so called because the high priest wore it only when he went to consult the Divine Majesty, was made of the same materials as the ephod; and being two spans in length by one in breadth, it formed a square when doubled. The span, he says, was half a cubit, or about ten inches.
I then begged of my uncle to explain the nature of the Urim and Thummim. He told me that the words signify light and perfection; but as Moses does not appear to have received directions for making them, it is impossible now to form any distinct idea of the materials of which those sacred ornaments were composed, or of the manner in which they were employed, in order to obtain answers from the mercy-seat in the Tabernacle. The opinions of the learned have therefore been very various on those points: the Jews think they consisted of precious stones, which were so arranged that the partial brilliancy of certain characters engraved on them pointed out the required reply. Others suppose that they were merely parts of the grand dress, which qualified the high priest to present himself in the holy place on great occasions. But the question is of little importance to us; like many other mysteries attending the Divine ordinances, we vainly endeavour to penetrate their meaning: we may, however, feel assured, Bertha, that if these things were necessary to be understood by us, they would have been fully explained. Many ceremonies in the ritual given to the Israelites, were adapted to them as a people who had lived amongst the heathens, and who had imbibed those prejudices and depravities of heathen worship, which were so totally removed from every thing spiritual. To us they may be objects of rational curiosity; but a knowledge of their use or precise fabric could add no essential testimony to the well-established truths of Scripture History.
“There is, however, one mode of viewing the subject, from which we may derive a useful hint: the high priest could not address the Almighty when divested of this emblem of light and perfection; in like manner our addresses to God will be of no use, unless we also are adorned, not indeed with the emblem of light, but with the true light of the Gospel; with that clear and bright faith which makes us feel the power and goodness of Him to whom we pray.”