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may be necessary to make and break electric contacts. The reduction of the arc a may be equally effected by either of two methods—either by reducing the impulse weights, or by shortening the duration of the impulse. When the arc is considerable, the former method appears preferable; when it is very small, there is not much to choose between the two; except that, by constantly reducing the impulse weights, they may perhaps become inconveniently small.
To return to the subject of compensation for variations of temperature, it may be observed that, while every pendulum is liable to be disturbed by the forces other than gravity acting upon it, and while these forces are not all of them subject to law, so that their effects can be exactly predicted and allowed for, it is not surprising that methods of compensation theoretically good should have failed to satisfy in practice. In the electric clock here presented, should its performance accord with expectation, and should it be found practicable to reduce the arc of vibration as far as it is at present believed to be, there will evi. dently be no sensible change of rate arising from any cause whatever, except expansion or contraction. If then the rate does actually change, the cause of error will be directly indicated; and the proper mode of correction 'may be made a subject of intelligent study.
A doubt has been already intimated above, whether the com. plaint made of the performance of the mercurial compensation, and of the glass jars as connected with it, is well founded. If the pendulum rod descends into the mercury, it would seem that there could be no great difference in the fluctuations of the tem. perature of the two metals. As the changes come from without it will be the rod which will be most directly exposed to them; but the capacity of the mercury for heat is so much less than that of steel, that its changes take place with correspondingly greater rapidity. If the smaller bulk of the rod in proportion to its surface, be in its favor, the remedy would be to make the rod larger, or to dispose the mercury in an annular vessel. But, at any rate, it is easy to make the containing vessel of iron accord. ing to the plan of Mr. Dent; and if this is done, and the expedient last suggested is adopted, of introducing the mercury into the annular space between two cylinders, it would seem that the mercurial compensation might be made quite perfect. As a final security against irregularities in the receiving of heat or parting with it, the entire surface both of the rod and of the containing vessel might be made uniform in character: which is done in the present clock by gilding. For a comparison of the performance of the compensation in glass and in iron, different jars are provided, which will be substituted for each other at intervals of several months. A brass cover, externally gilded, is also
provided, to be placed over either the iron or the glass jar, for the purpose of observing the effect of change of external surface.
These are some of the arrangements which have been made for future use in the experimental examination of the question under consideration. They would not have been brought to the notice of the Association until after having been instrumental in securing some results, were it not for the fact that no other opportunity will occur of exhibiting the clock itself—its comple. tion having taken place just as the Association are meeting-and the constructor being on the point of forwarding it to the University of Mississippi, where it belongs. The observatory of the university is now in progress of erection, and it will be some time before the large transit instrument which is to be provided will be set up. It will be practicable, however, with less perfect facilities, to make some of those observations for which this clock is designed; and the conclusions to which such observa. tions may lead will be communicated hereafter.
ART. XXIII.-Enumeration of Ferns collected by Mr. Charles
Wright, in Eastern Cuba in 1856–7; by DANIEL C. EATON.
This enumeration has been prepared for the benefit of the subscribers to Mr. Wright's Cuban collections. It is unavoidably imperfect, since my materials for identifying tropical ferns are scanty, and, indeed, I should not venture to print it, were it not for the kindness of Sir W. J. Hooker, who has examined and named for me several of the more obscure species. Mr. Wright is again collecting in Cuba: after his return a supple. ment to this list will probably be published.
774. Hemionitis palmata, Linn. 775. Antrophyum subsessile, k'unze, Analect. p. 29, t. 19. 776. A. lanceolatum, kaulf. 777. Gymnogramme tartarea, Desv. 778. G. sulphurea, Desv. 779. G. trifoliata, Desv. 780. Xiphopteris serrulata, Kaulf. 781. Meniscium sorbifolium, Willd.; Langsd. and Fisch. Ic. Fil. t. 4.
782. M. sorbifolium, Swartz. This is probably but a variety of the last with narrower pinnæ.
783. Gymnopteris aliena, Presl. (Acrostichum alienum, Swartz.)
784 & 785. Olfersia cervina, Kunze; Hook. Filices Exoticæ, t. 43. (Acrostichum cervinum, Swartz.)
786. Polybotrya osmundacea, Humb. & Bonpl.?
787. Lomariopsis sorbifolia, Fée, Hist. des Acrostichacées, p. 69, var. ? Perhaps this Fern deserves to be described as a new species, but I am un. willing to name and describe it from my present scanty materials. SECOND SERIES, VOL. XXVII, No. 80.—MARCH, 1859.
788. Gymnopteris nicotianæfolia, Presl. (Acrostichum nicotianæfolium, Swartz.)
789. Elaphoglossum ciliatum, T. Moore, Index Filicum, p. 8. (Acrostichum Preslianum, Fée, 2me. Mém. p. 46, t. 24; Hook. in litt.)
790 & 791. E. latifolium, J. Smith, Catal. Kew Ferns, p. 3. (Acrostichum latifolium, Swartz; Hook. Fil. Erot, t. 42.)
The name Elaphoglossum is retained for this genus because there was no representative of it in the original genus Acrostichum, (Linn. Amon. Acad. i, p. 268, which contained only two real Acrostichaceæ, A. aureum and A. lanceolatum (Leptochilus Linnæanus, Fée), the former of which must keep the name Acrostichum.
792. Hymenodium crinitum, Fée, 2me. Mém. p. 90. (Acrostichum crinitum, L.; Hook. Fil. E.rot. t. 6.)
794. E. piloselloides, T. Moore, I. c. p. 13. (Acrostichum piloselloides, Presl.; Hook. I. c. t. 29.)
795. Goniophlebium 'incanum, J. Smith. (Polypodium incanum, Swartz.)
796. Pleopeltis angustifolia. (Polypodium elongatum, Mettenius, Po lypod. p. 88, non Pleopeltis elongata, Kaulf.)
797. Campyloneuron tæniosum, Fée, Gen. Fil. p. 258. (Polypodium tæniosum, Willd.; Mettenius, l. c., p. 52.)
798. Goniophlebium piloselloides, J. Smith, in Hook. Jour, Bot. 4, p. 56. (Polypodium piloselloides, L.)
801. C. Cubense, Fée, Iconogr. p. 14 and 129, t. 3. (Polypodium tæ, niosum, var. Mettenius, l.c.)
803. Phlebodium aureum, R. Br.;. Hook. Gen. Fil. t. 112. (Polypodium aureum, L. Chrysopteris aurea, Link, Fil. sp. p. 121.)
804. Goniophlebium neriifolium, J. Smith, l. c. (Polypodium neriifolium, Swartz.) Hook, in litt.
805. Polypodium sororium, H. B. K. 806. P. pectinatum, L. 807. P. Funiculum, Fée, Iconogr. p. 12, t. 18. 808 & 810. P. suspensum, L.; Hook. in litt. 809. P. Camptoneuron, Fée, Gen. Fil. p. 237, Iconogr. p. 60, t. 23. 811. P. trichomanoides, Swartz. 812. P. hastafolium, Swartz ; Hook. & Grev. Ic. Fil. t. 203. 813. Goniopteris reptans, Presl. (Polypodium reptans, Swartz.) 814, 816 & 865. Polypodium (Phegopteris) sanctum, Swartz. 815. Lastrea pubescens, Presl. (Aspidium pubescens, Swartz; Vid. Hook. & Grev. Ic. Fil. t. 162.
817. Goniopteris tetragona, Presl. (Polypodium tetragonum, Swartz ; Schkuhr, Fil. t. 186. Phegopteris tetragona, Metten. Fil. Lips. p. 84.) Hook. in litt.
818, 819 & 822. Lastrea patens, Presl. (Aspidium patens, Swartz.) This is a common and most variable fern in the Southern States from Florida to Louisiana and Texas. It resembles Nephrodium molle, and was mistaken for that species by Kunze. (Am. Jour. Sci. vi, p. 83.)
820. L. contermina, Presl. (Aspidium conterminum, Willd.) Hook. in litt,
823. Nephrodium deltoideum, Desv. (Aspidium deltoideum, Swartz ; Metten, Phegopt, und Aspid. p. 93.)
824. N. Skinneri, Moore, Index Filicum, p. 104. (Aspidium Skinneri, Hook. Ic. Pl. t. 924.) ?
825. N. stenopteris. (Aspidium stenopteris, Kunze, Fil. 2, p. 48, t. 120.)
826. Nephrolepis exaltata, Presl.
827. Goniophlebium loriceum, Fée, Gen. Fil. p. 255. (Polypodium loriceum, L.)
829. Polystichum triangulum, Fée. var. (Aspidium triangulum, L. var. laxum, Hook. Fil. Erot. t. 33. Polystichum ilicifolium, Fée. Gen. Fil. p. 279, Iconogr. p. 21, t. 6.
830. Lastrea Melanochlamys, Moore, l. c. p. 96. (Aspidium Melanochlamys, Fée, Gen. Fil. p. 294.)
831. L. exculta, Moore, l. c. p. 91. (Aspidium excultum, Metten. Phegopt. und Aspid. p. 69. Aspidium lætum, Moritz.) Hook. in litt.
832. Polystichum platyphyllum, Presl. (Aspidium platyphyllum, Willd. Phegopteris platyphylla, Metten, 1. c. p. 122.)
833. Aspidium cicutarium, Swartz ; Metten. 1.c. p. 117.
838. H. marginatum, L.; Hook. l. c. t. 73. (Hemidictyum marginatum, Presl.)
840. A. serra, Langsd. & Fisch. Ic. Fil. t. 19.
842. A. dimidiatum, Swartz. (A. zamiæfolium, Kunze, Fil. p. 103, t. 48.)
833. A. falcato, Lum., affine.
844. Fadyenia prolifera, Hook. Gen. Fil. t. 58, B; Fil. Exot. t. 36. (Aspidium Fadyenii, Mettenius, Fil. Hort. Lips. p. 95. Asplenium proliferum, Swartz.)
845. Asplenium, salicifolio, L. affine.
858 & 859. Onychium strictum, Kunze, Fil. 2, p. 11; Hook. Sp. Fil. 2, p. 123.
860. Gymnogramme leptophylla, Desv.
864. Lomaria decresceus, Fée, Gen. Fil. p. 68, Iconogr. p. 24, t. 9. (L. attenuata, Willd., ex Hook. in litt.)
865. Vittaria lineata, Swartz—(the longer specimens.) 865 bis. V. sp. ign.—(the shorter specimens.)
866. Pleurogramme immersa, Fée. 3me. Míém. p. 37, t. 4. Hook. in litt.
867. Pteris pedata, L.; Hook. Sp. Fil. 2, p. 208. Fil. Erot. t. 34. (Doryopteris pedata, J. Smith.)
868. P. leptophylla, Swartz; Hook. Sp. Fil. 2, p. 216. (Litobrochia leptophylla, Fée, Gen. Fil. p. 135.) *869. P. mutilata, L.; Hook. I. c. p. 164, t. 131.
870. P. denticulata, Swartz; Hook. I. c. p. 215. (Litobrochia denticu·lata, Fée, l. c.)
871. P. longifolia, L.
873. P. aculeata, Swartz; Hook. I. c. p. 224. (Litobrochia denticulata, Fée, l. c.)
874. Adiantum macrophyllum, Swartz ; Hook. Fil. Exot. t. 55.
886. Polypodium (Phegopteris) barbatum, Kunze in Linnæa, 9, 52. Hook, in litt.
887. Cheilanthes microphylla, Swartz.
888. Hemitelia horrida, R. Br.; Hook. Sp. Fil. 1. p. 30, t. 15; Fil. Exot. t. 69.