little farther along the street to its terminus, | wood connected with the original battery, but which was formed by the solid-arched entrance of Fort Triana, flanked on either side by massive buttresses, which were pierced by occasional narrow embrasures, and surmounted by circular sentry towers. The pavement was completely grass-grown and the whole structure was in a luxuriantly-vegetating condition. Its great solidity and strength first attracted our notice. The inclosing walls could scarcely have measured at their base less than from ten to twelve feet in thickness, and were, on the sea-side, at least forty feet in height. The paved floor of the interior was covered over by a tangled growth of vines, principally of a species of the mimosa popularly known as the "sensitive plant," whose delicate foliage shrank into apparent leaflessness as we forced our way along. Several heaps of rust-caten bomb-shells and cannon-balls were visible through the interlacing vines; and upon an elevated part of the fortification, which was reached by a sort of stone inclined plane, were lying about a dozen immense cannon. Time had finished his work with carriages of the guns and every thing of the guns were as smooth and polished as if fresh from the foundry. This was, as our good missionary informed us, in consequence of the large quantity of silver used in their manufacture. He said that their value, for the silver alone, was very great. Among the trees, which on portions of the fortress might almost be said to form groves, some were of very curious and even fantastic shape. One, which had taken root in the interior of a watch-tower, had grown to an enormous size, and burst the inclosing walls, carrying up several large stones embraced in its branches; another, springing from the base of the wall near a narrow musket-embrasure, had passed through it, and developed to at least eighteen inches in diameter both on the outer and inner side of the wall, but in its passage through the opening it was no more than about two or three inches in thickness. During our explorations about the fort an entrance into a dark arch-way was discovered, supposed to terminate in the ancient powder magazine. Consenting to a proposal from one of the party to examine it, we groped along a low, has been bled by a vampire, he is chosen in preference to others equally exposed for subsequent attacks, and though he remove ten, and even twenty miles away, no immunity is gained thereby, but each morning awakens weaker and weaker un damp passage for some half a dozen yards, and | til he either succumbs or the animal is discov came to an open space, which, by swinging our umbrellas, was found to be an arched chamber about thirty feet square. The size and shape of the place, however, was not ascertained before finding that it had other occupants than ourselves. Multitudes of immense bats of the vampire species were domiciled there, and seemed by no means disposed to vacate the premises. A sharp skirmish was the result, and we came out with divers bites and bruises and a couple of specimens, one of which, with wings outstretched, measured over twenty inches in breadth. Imagine a large mouse, with a horn upon its nose like a rhinoceros-furnish him with a pair of demoniac wings, and you have a very fair picture of the vampire. He is dreaded by the natives on account of his blood-sucking propensities. In the sultry tropical nights he fans the heated sleeper with his wings while his needle-like teeth are being inserted into the veins of his victim, when he slakes his thirst with almost inconceivable gentleness, and it is only by some fortunate chance that he is ever discovered before the mischief is done. It is a curious and well-authenticated fact, that once an individual ered and killed. Cattle and horses, from being more exposed, are more frequently the subjects of attack. Passing out of the fortress we retraced our steps along the street, and continued until the opposite end of the town was reached. Here we found the ruins of a once stately cathedral, which time and neglect had marked as belonging to the past; its crumbling tower still held two or three of the bells of its ancient chime, but the rest had fallen, and were lying halfburied in the earth at its base. There was also the ruins of a marble colonnade near by, a few of its pillars still upright, telling where some princely edifice had stood. At the extremity of the street a large iron cross, mounted upon a massive stone foundation, indicated the starting-point of the great paved road which led to Panama in the olden time. This cross is represented in the cut which heads this paper. On every side of the stone-work were little arched niches in which it was the custom of travelers to deposit votive offerings previous to starting upon a journey to the Pacific Coast. It was still held in reverential esteem by the natives, several of whom were kneeling before it as we passed; | of soldiers along the path. As we watched the and in some of the niches were sea-shells filled with fish or cocoa-nut oil, in which little tapers were burning. The ancient road was about ten feet in width, paved with large cobble-stones, and still in a remarkable state of preservation. It was covered with short velvety grass, except in the middle, where a tiny ant-path was worn. Extending along it as far as the eye could reach, the treasure-freighted mule-trains and the gay cavalcades that in the long ago" had made busy clatter along its course, were now represented by myriads of little ants laden with leaves, and marching with the order and precision of a line operations of these little creatures, the ingenuity displayed in carving out their burdens from the foliage by the road-side-the regularity and activity with which they filed into the road, and plodded their way to the hillocks near the old cross, to which we traced them-the thought suggested itself that if the original projectors of this great paved way had taken lessons from these prudent and industrious insects, and, like them, had been peaceable and persevering-depending upon the rich products of home production, instead of grasping for far-off and uncertain wealth-they might still have been the possessors of the place, happy and prosperous. But THE MICROSCOPE. MOST OST persons, I imagine, must have seen little children pick currants and citron out of a cake, and leave the bread part untouched. Even thus would it be with some of my gentle readers, perhaps, if I were not on my guard; but whoever eats of my cake shall eat fairly. The observer who looks at a microscopic object through that magic tube, the microscope, for the first time, is so delighted-I may say enraptured-with the wonderful visions made evident, that he would like nothing better than for some one to take pen and pencil in hand, and, without prefacing one word about the nature of the microscope, begin to give pleasing illustrations. I don't approve of people picking currants and citron out of my cake in that way. Some people I have met with are dreadfully shy of encountering a math cult. Now I can not stir one step in the way of teaching the nature of the microscope until my readers apprehend the meaning of the proposition, "that the apparent magnitude of bodies is proportionate to the size of the angle they subtend on the seeing part of the eye." the Spaniards never thought of developing the | ematical term, thinking it must be dry and diffigreatest riches of the country: always coveting and warring for gold, they fought and grasped until its possession drew upon them a merited punishment; while the hills and valleys of the land, with their wondrous vegetative wealth, that alone would have given a great and lasting prosperity, remained uncared-for and undeveloped. Somewhat fatigued with our wanderings, we adjourned to the inviting shade of a large orange-tree by the road-side, and partook of a generous luncheon, making our dessert off the luscious fruit with which the branches above us were filled. After the repast we strolled along to the upper shore of the harbor, and visited the remains of several government store-houses and another large fortification, which, however, possessed neither novelty nor interest enough to be worthy of further mention. Large numbers of pelicans were quietly perched upon the trees overhanging the beach, making an occasional splashing descent upon the water, as some unlucky passing fish attracted their attention. They were so tame that we walked directly under their roosting places without causing any disturbance. Our jaunt was here terminated by the report of a signal-gun from the ship, giving notice that the watering was completed. So, hastening back to the landing-place, we bade our good friend the missionary adieu, and embarking, pushed off for the ship-the worthy "Terence," halfleg deep in the water, shouting after us all sorts of good luck and speedy return. The night was just closing around as we arrived on board, and every thing being in readiness we at once steamed out of the harbor, and arrived safely alongside the wharf at Navy Bay at about 10 P.M.; thus satisfactorily completing our excursion to the ancient harbor and city of Porto Bello. Ladies don't study Euclid, and I don't wish them; but I trust that many a lady will read what I am now writing. For their special aid, therefore, I beg to intimate that an angle is a corner. For example, the corner in the lower part of the letter V is an angle; and if the two legs of the V were to be prolonged ever so far, the angle would be none the bigger in a mathematical sense. When we speak of an angle being large or small we do not mean that its legs are long or short, but that the corner is blunt or sharp. The sharper the corner, the smaller is the angle; the more blunt, the larger it is. Follow me now to my Dutch clock-we shall find it useful. The hands of my clock are out of order, and if I do not tighten them on their pivots they slip and move about in most eccentric fashion. Let us turn the eccentricity of my Dutch clock to account. Fixing the hourhand at XII., and removing the wedge by which it is tightened on the pivot, the obedient hand will stay pointing at XII., though I cause the minute-hand to move quite round the dial. Very well. Fancy now the circular part of the dial to be divided into 360 equal parts; then, if I point the minute-hand to seven minutes and a half past XII., it will be evident, if you count, that the minute-hand proceeding from XII. will have traversed over 45 of the parts into which the circle is divided, and will be said to form an angle of 45 degrees with the hour-hand. If the minute-hand be pushed on to III. it will have passed over 90 of the equal parts or de- | ments of the former kind are called telescopesgrees, and will form the angle of which a square of the latter kind, microscopes: both are instrusurface has four, and which mathematicians call ments having the property of increasing the size a right angle. If I have occasion, then, here- of visual angles. after to state that an angle is one of a certain number of degrees, you will know exactly what I mean. Now imagine yourself standing with your face toward an object-say a tower—from the very highest and the very lowest part of which a thread proceeds, the two extremes of the thread meeting in one of your eyes; then it follows that the nearer you are to the object, the larger will the angle be which the threads make. The following diagram will render this evident in a moment. Now every body knows that the farther a spectator removes from any object the smaller does the object appear, until at last it ceases to be visible altogether; the fact being, that the unaided human eye can not perceive an object under a smaller visual angle than three degrees. If, then, we could manage to convert a small visual angle into a large one, an object too far off, or too naturally small to be visible, might be rendered visible; for the reader will be good enough to understand that our assumed pieces of thread are tangible representatives of visual rays of light. In combining different glasses, whether to form a telescope or a microscope, enormous difficulties had at first to be overcome-so great, indeed, that the illustrious philosopher, Newton, gave up the task in despair. Not only is much light sacrificed by passing through numerous glasses, and objects, though magnified, are rendered indistinct, except special care be taken; but the light which is transmitted does not appear of its true color, except special provision be made for overcoming what is termed spherical observation. In the manufacture of tele scopes, mirrors were for a long time employed, to a great extent, instead of glasses, for avoiding this defect, and mirror-microscopes were, indeed, also made; but they so little answered the purpose intended that philosophers abandoned them in favor of the single microscope, as it was called-an instrument consisting only of one single magnifying glass. All the wonderful discoveries of the early microscopists were effected by instruments of this kind, the triumph of modern microscope makers being, that they have succeeded in combining various glasses, still preserving the true colors of the object viewed. Certain kinds of glass are false for one color, and certain kinds for another. One kind of glass will disperse red light, another yellow, and a third blue light; but, by combining all three kinds of glass together, the imperfections of each may be neutralized, and objects seen in their proper colors. To the ordinary observer a piece of glass is a piece of glass and nothing more; not so to the microscope-maker. To him the exact power of refraction or bending, which each piece of glass possesses for light of different colors, is of the utmost importance. Varieties of English glass answer well for some of the lenses entering into a microscope; but for a certain kind of glass the microscope-maker has to send to Switzerland. Microscopic glasses, or lenses, are so troublesome to get into form that, though the material glass is cheap, the lenses become very expensive. Above all things, it is necessary that the original glass shall be the best of its kind. A common observer would not discriminate any difference between various samples; but the practiced eye of the microscope-maker is quick at perceiving imperfections. He places each piece of glass on a little globule of mercury, and notices the appearance presented by the shining metal when viewed through it. If it be not distorted-if the light comes regularly through de---the glass is presumed to be good; otherwise, It it is rejected as unfit. Certain glasses, I need not say, have the property of making objects appear large, whence they are called magnifying glasses; and, if what I have already stated be correct, their magnifying power is due to the property which they have of converting small visual angles into those which are larger. That is to say, they bend or refract the rays of light as represented in the following diagram, making them converge to a point sooner than they otherwise would. T' Observe, the piece of glass represented in my picture above, at G, is not flat; it bulges out on either side, and forms a thin edge all around. It is this peculiar conformation which gives it the magnifying power; why or wherefore would take me too far into the science of optics to scribe just now; such, however, is the fact. is possible, therefore, by means of glasses, to render large objects visible, which would otherwise be invisible because of their distance; and small things visible, which would be otherwise invisible on account of their smallness. Instru We have already seen that the sides of a magnifying glass bulge out. Different degrees of bulging are imparted, to suit the exact conditions aimed at-the bulging or convexity of some lenses being more considerable than of others. |