306 Linnaan Society. 



other fluid, to the height of two or three inches ; while the remaining 

 parts of the tube, together with the cistern, to the depth of about half 

 an inch, are filled with oil ; care being taken to bring the surfaces of 

 water in both legs to the same level, by equalizing the pressure of the 

 incumbent columns of oil. If the horizontal pipe be applied to the 

 key-hole of a door, or any similar perforation in a partition between 

 portions of the atmosphere in which the pressures are unequal, the 

 fluid in the corresponding half of the instrument will be depressed, 

 while it is raised in the opposite one, until the excess of weight in the 

 column that is elevated will just balance the external force resulting 

 from the inequality of atmospheric pressures upon the surface of oil in 

 both cisterns. This, however, is equal only to the difference between 

 the weight of the column of water pressing on one side, and that of 

 an equal column of oil which occupies the same length of tube on the 

 other side : this difference, depending upon the relative specific gra- 

 vities of the two fluids, will, in the case of olive-oil and water, be about 

 one-eleventh of the weight of the column of water elevated. But the 

 sensibility of the instrument might be increased at pleasure, by mix- 

 ing with the water a greater or less quantity of alcohol, by which the 

 excess of its specific gravity over that of the oil may be reduced to 

 one-twentieth, one-thirtieth, or any other assignable proportion. The 

 instrument may be converted into an areometer, by closing both the 

 cisterns, and by applying to the upper part of each a trumpet-mouthed 

 aperture, opening laterally. 



Feb. 12. — The President in the chair. A paper was read, intitled, 

 " On the Reflection and Decomposition of Light, at the separating 

 Surfaces of Media of the same and of diff^erent Refractive Powers," 

 by D. Brewster, M.D. F.R.S., &:c. 



Feb. 19. — The President in the chair. — A paper was read, intitled, 

 " Considerations of the objections raised against the geometrical re- 

 presentation of the square roots of negative quantities," by the Rev. 

 J. Warren, M.A. of Jesus College, Cambridge. Communicated by 

 Dr. Young. 



linn.s;an society. 



March 3. — A continuation was read of Mr. Don's paper on the 

 Composite' of South America. 



March 17. — A paper was read on the Fig-trees of Jamaica, by 

 James Mac Fadyen, Esq., Island Botanist : communicated by H.T. De 

 la Beche, Esq., F.R.S., &c. — This paper describes six species indi- 

 genous to Jamaica. The genus is divided into two sections, accord- 

 ing as the fruit is sessile or pedunculated. Under the 1st are enu- 

 merated F. Simpsoni and F. cordifolia; under the 2nd, F. Jamai- 

 censis, F. viridis, F. Americana, and F. lentiginosa. The author states 

 that the first four species are new, and that the characters of the rest 

 had never before been properly investigated. 



Some remarks by Mr. Bicheno on the geographical and geological 

 distribution of Plants were also read : and the reading of Mr. Don's 

 paper was continued. 



FRIDAY 



