SgO PHILOSOPHICAL TRANSACTIONS. [aNNO 1733. 



to another; and therefore, that when they leap they are observed to be pendu- 

 lous in the air; but when they have done leaping, they are found to hang from 

 trees. Besides, these vespertiliones admirabiles may be called feles volantes 

 with equal propriety, as Gesner called the sciuri here spoken of, volantes. 



A Description of a Barometer, of which the Scale of Variation may be increased 

 at Pleasure. Bij the Rev. John Rowning.* N° 427, p- SQ- 



ABCD, fig. b, pi. 15, represents a cylindrical vessel, filled with a fluid to the 

 height w, in which is immerged the barometer sv, consisting of the following 

 parts: the principal of which is the glass tube tp, represented separately at tp, 

 whose upper end t is hermetically sealed : this end does not appear to the eye, 

 being received by the lower end of a tin pipe gh, which in its other end g 

 receives a cylindrical rod or tube sx, either hollow or solid, and made of any 

 materials, whatever, fixing it to the tube tp. The rod sx may be taken off, to 

 put in its stead a larger or smaller, as occasion requires, s is a star at the top 

 of the rod st, which serves as an index, pointing to the graduated scale la, 

 fixed to the cover of the vessel abcd. mn is a large cylindrical tube of tin, 

 represented separately at mn, which receives in its cavity the smaller part of the 

 tube tp, and is well cemented to it at both ends, that none of the fluid can 

 get in. 



The tube tp, with this apparatus, being filled with mercury, and plunged 

 into the basin v, which hangs by two or more wires on the lower end of the 

 tube MN, must be so poised as to float in the liquor contained in the vessel 

 ABCD, and then it will rise when the atmosphere becomes lighter, and e 

 contra. 



Let the specific gravity of quicksilver be to that of water, or to the liquor 

 the barometer floats in, as * to 1 : and if it be proposed that the variations of 

 this compound barometer shall be to the contemporary variations of the com- 

 mon barometer, in the given ratio of n to 1 ; this efl^ect will be obtained by 

 making the diameter of the rod st, to the diameter of the cavity of the tube 

 HI, as v^ - — ^ to 1 ; which may be thus demonstrated. 



Let us suppose that the variation of the height ot the quicksilver in the 

 common barometer, called v, is such, that a cubic inch of quicksilver shall rise 

 into the vacuum xt; in order to which, a cubic inch of quicksilver must rise 



* Mr. Rowning was an ingenious mechanist, mathematician, and philosopher. He published a 

 very neat " Compendious System of Natural Philosophy," in 2 volumes, Svo. 17+4-; also Descrip- 

 tion of a Machine for finding the Roots of Equations, in 4to. 1771. Mr. Rowuing was rector of 

 Anderby in Lincolnshire; and he died in 1771, at 72 years of age. 



