(By the particular request of a "Mechanic" in Cherryfield, Me.)--In this art the process is various according to the circumstances, and the ground on subjects to which it is applied. In painting common chairs, the ground is prepared by a coat of paint composed of ivory black and rose-pink,--equal quantities, ground in a mixture of equal parts of linseed oil, drying japan and spirits of turpentine.--When this is dry, the graining color, consisting of three parts of rose-pink with one of vermillion, ground in a mixture of oil, japan and spirits of turpentine, is applied with a common flat graining brush. Fancy boxes and cabinet furniture are painted by a different process, by which a better imitation is produced. The ground is prepared by one or more coats of white lead changed two or three shades with yellow ochre. When dry, a thin staining of burnt terra-de-sienna ground in water, containing a very little sugar or gumarabic is laid on the work, and while this continues moist and flowing, the graining is applied. The graining should consist of a mixture of black and rose pink, ground in the staining compound. This must be varnished when dry, with copal varnish. Some prefer, however, to grind the staining and graining in oil, diluted with spirits of turpentine. The learner must have some sample pieces of varnished rosewood before him when graining.
[India Rubber.]
The substance called India Rubber, or Caoutchouc, was not known in Europe until the beginning of the eighteenth century. It was originally brought as a great curiosity from South America. Europeans continued ignorant of its origin until a deputation of the French Academicians undertook a voyage to South America in 1735, for the purpose of obtaining the correct admeasurement of a degree of the meridian. These philosophers did not confine their attention to the one great object of their pursuit, but among other interesting discoveries made themselves acquainted with that peculiar substance--caoutchouc. These Academicians discovered at Emeralds, in Brazil, trees called by the natives heve, whence flowed a juice, which, when dried, proved to be what is called India Rubber. The heve was also found growing in Cayenne, and on the banks of the Amazon river. It has since been discovered that caoutchouc may be obtained from another species of tree growing in South America, called jatropha elastica. If these trees are punctured, a milky juice flows out, which, on exposure to the air, thickens into a substance of a pure white color, having neither taste nor smell. The hue of the caoutchouc of commerce is black in consequence of the method employed in drying it. The usual manner of performing this operation is to spread a thin coating of the milky juice upon the moulds made of clay, and fashioned into a variety of figures. These are then dried by exposure to the heat of a smoke-fire: another layer is then spread over the first, and dried by the same means; and thus layer after layer is put on, until the whole is of the required thickness. While yet soft it will receive and retain any impression that may be given to if on the outside. When perfectly dry the clay within is broken into small fragments by percussion, and the pieces are drawn out through the aperture which is always left fur the purpose. The common bottle of India Rubber, therefore, consists of numerous layers of pure caoutchouc, alternating with as many layers of soot.
The natives of those parts of South America to which these trees are indigenous, convert the juice to a variety of purposes. They collect it chiefly in the rainy season, because, though it will exude at all times, it flows then most abundantly. Boots are made of it by the Indians, through which water cannot penetrate; and the inhabitants of Quito prepare a kind of cloth with it, which they apply to the same purposes as those for which oil-cloth or tarpaulin, it used here. This, no doubt, is similar to the cloth now prepared with this substance in America, the use of which yields so many important advantages.--Youths' Gazette.
[Communication on Atmospheric Resistance.]
The following letter has been on hand several weeks, but deferred on account of a constant press of matter by which the limited space in our former small sheet was crowded. Our respected correspondent has consented to excuse the delay.
Providence, ---- 1846.
Friend Porter: In January last, I addressed a few lines to you, asking information in regard to an article entitled Atmospheric Resistance, in the New York Mechanic, of December 11, 1841. In your answer, you say if the full surface is 30,000 square feet to each wing, (which makes 60,000 square feet,) only about half of one horse power would be required to sustain this weight, and I understand you, virtually to say, that they must be ten times as large, in order that the strength of one man be sufficient to work this and elevate himself together with the apparatus, if it were not too heavy. Now, this makes 600,000 square feet. This is rather more than 774 feet square: rather large sized wings. One would suppose that they might lift rather heavy, if they were very light, being 387 by 774 feet each. Now, to me this is entirely incomprehensible, and I should like an explanation, if this calculation is correct, how it is that an eagle which sometimes weighs nearly thirty pounds, can elevate himself, with so much ease, and even carry with him nearly his own weight, using a pair of wings, which if they were five feet long and two feet wide each, would make but twenty feet of surface. Thus, you will see, is no where in proportion to the weight even of the eagle alone, (which we will suppose to weigh twenty pounds,) that the wings bears to the 150 pounds, while on the other hand, it is near in proportion to the surface of the wings of a pidgeon and its weight. Nor can I comprehend why it would require so much power, the eagle though he exerts himself considerable in rising, no doubt, does not seem to use power any where in the proportion that you have thought would be required supposing the wings to be made in the same proportion to the 150 pounds that his wings are to his weight, his beats are not so quick but what they can be very easily counted.