SECTION IV. Alæ longæ, remigum pogoniis internis emarginatis. The Muscicapa Tyrannus of Linnæus (the Savana of Buffon) may be taken as the type of a division strongly characterized by the tail being unusually long and very deeply forked; this structure, aided by a corresponding power in the wings, enables these birds to fly with a rapidity even greater than that exerted by those of the second division; both groups approach each other in general conformation, but the different form of their tails, and a variation in their economy, seem to warrant a separation. The habits of the Tyranni in general are solitary; for, although several of the same species may be seen within a limited distance, they never act in concert; but the Tyrannus furcatus is in some degree gregarious; it shews a fondness for the society of its own species, and frequently assembles in the air in bands of near two hundred. Whether this is for the purpose of pursuing insects, or preparatory to migration, M. Vieillot (who quotes the authority of Azara) has not distinctly stated; but the fact itself is curious, and moreover points out a considerable resemblance to the gregarious habits of the African Drongos; the typical groups of which are no less remarkable for the great length of their tails. There are but few birds, yet known, referrible to this section. The variety of T. Savana, said to be found in Canada, may probably be distinct. The Muscicapa forficata of Latham is known to me only by the figure in Buffon, but from that it seems to possess the typical characters in great perfection; a fourth species, which I believe is here described for the first time, will complete the list. Sp. 17. TYRANNUS Savana. Vieil. Fork-tailed Tyrant. T. cinereus, albus infrà; vertice auribusque nigris; alis fuscis; caudâ longissimâ, valdè furcatâ, nigrâ. Cinereous, beneath white; crown and ears black; wings brown; tail very long, deeply forked, and black. Muscicapa Tyrannus. Lin. 1. 325. Ind. Orn. 2. 484. Le Tyran a queue fourchue. Brisson, Ois. 2. pl. 39. f. 3. Fork-tailed Fly-catcher. Gen. Syn. 3. p. 355. Tyrannus Savana. Vieil. Ois. Am. 1. pl. 43. Size of a lark; bill black, and smaller than that of T. intrepidus; sides, ears, and upper part of the head deep black, concealing a crest of bright yellow on the crown; general colour of the upper plumage light cinereous, changing to blackish on the rump: wings gray-brown, rather lengthened; the two first quills abruptly emarginated, or notched, very near their extremities; the exterior web of the first quill is pale yellow: all the under plumage is pure white. The tail is black and very long ; the exterior pair of feathers exceed the rest by nearly three inches, and are margined externally, to half their length, by pale yellow. The tarsi are short and black. Inhabits Brazil, where it is, however, rare; in other parts of South America it seems to be more common. Total length 111, bill, wings 44, tail 7, depth of the fork 44, tarsi. Sp. 18. TYRANNUS longipennis. Sp. Nov. Gray fork-tailed Tyrant. T. cinereus ; mento albente; caudâ fuscâ, longâ, furcatâ. Size of the last, but the bill is smaller and more depressed. The whole of the plumage, both above and beneath, is cinereous or slate-coloured; the feathers on the crown form an incumbent crest, and are obscurely streaked with blackish; the chin is nearly white. The wings are long, and of a uniform sooty-black; all the primaries are gradually pointed, but the second is particularly narrow, and the point acute. The tail is considerably lengthened, (though much shorter than that of the last,) deeply forked, and sooty-black: the exterior pair of feathers are nearly threequarters of an inch longer than the others, and are margined externally with white. Tarsi short, as in T. Savana. Discovered in Brazil by M. Natterer, Zoologist to the Austrian Government; but I am not aware of its being yet described. Total length 94, bill, wings 44, tail 5, depth of the fork 14, tarsi T In this attempt to characterize the leading groups among the Tyranni, it will be seen that their distinctions principally rest on characters hitherto considered of little importance. I have therefore found it necessary to give detailed, and often minute, descriptions of those species, whose identity was in any way questionable. Had this not been done, the modifications by which nature gradually passes from one form to another, could not have been traced. It must also be remembered, that the clear elucidation of species more particularly concerns the application of Natural History to the practical purposes of life; while nothing will more contribute to aid the general views of those naturalists, who study the grander operations of nature. varied with white; beneath white, with brown stripes Olive, beneath yellow; throat cinereous; wings and tail externally rufous Olive-brown; knees armed with spines Gray-brown, beneath yellow; first primary quill pointed ditto Primary quills emarginate and pointed. Tail even. Olive-gray, bill small, chin white; tail black Tail 12 Gray-brown, beneath yellow; throat cinereous; first primary obsoletely pointed ferox Cinereous, beneath white, tail forked forked. 'Olive, beneath yellow; head and neck cinereous Olive-brown, beneath yellow; crest orange; tarsi strong Gray, chin white, tail brown Grey, beneath white; wings and tail black, varied with white Cinereous, beneath white; crown and ears black griseus crudelis 316 Astronomical and Nautical Collections. may be obtained from the following rule, given in the first edition of the requisite Tables. As 400 is to the difference of the thermometer from 55°, so is the mean refraction to its correction, to be subtracted, if the thermometer is higher than 55°; but to be added, if the thermometer is lower. As 300 is to the difference between the altitude of the barometer and 30 inches, expressed in tenths of an inch, so is the refraction corrected for the thermometer to the correction on account of the barometer, which added to or subtracted from the refraction corrected for the thermometer, accordingly as the barometer is higher or lower than 30 inches, gives the true refraction corrected on account of both. In this example 25′′ and 3′′ are the respective variations of the sun and moon's refractions, and 4" the variation of the auxiliary. angle, all to be added, as the true refraction is less than the mean. Astronomical and Nautical Collections. iii. Observations on Mr. Mendoza y Rios's Method of True from the Apparent LUNAR DISTANCES. HENDERSON, Esq. 315 computing the By THOMAS It is a general opinion among seafaring persons that the method of Mr. Mendoza y Rios, for computing the true lunar distances, by means of his Tables for Navigation and Nautical Astronomy, the second edition of which was published at London, in 1809, is, for nautical purposes, the most easy, concise, and accurate. But the ingenious author not having fully shown how allowance is to be the made for the variation of refraction, arising from a change in pressure and temperature of the atmosphere, indicated by the barometer and thermometer, and for the effect on the moon's paral lax, occasioned by the spheroidal figure of the Earth, it may be proper to supply this deficiency. As explained by the Author, Tables VI., VII., VIII., and IX., showing the corrections of the apparent altitudes of the sun, stars, and moon, and the auxiliary angles for the computation of the true distances, are constructed for that state of the atmosphere, when the barometer is at 29.6 inches, and Fahrenheit's thermometer at 50 degrees. For any other weight and temperature, the variation of refraction is to be added to or subtracted from the corrections in Table VI., accordingly as the true refraction is greater or less than the mean; but to the corrections in Tables VII. and VIII., and the auxiliary angles in Table IX., the variation of refraction is to be applied in a contrary manner; that is to say, to be added or subtracted, according as the true refraction is less or greater than the The variation of refraction for the auxiliary angles in Table IX. is in every case the same as the variation for the mean refraction 66 seconds, which corresponds to the apparent altitude 40° 40. After these corrections are made, the true distance is computed as usual. mean. The Author, in his explanation of Table VI., has shown how the true refraction is to be calculated. A Table, such as the eighth in Mackay's Treatise on the Longitude, would be useful for exhibiting the changes of refraction at sight. Or the true refraction |