250 MR. CLERK MAXWELL ON THE VISCOSITY 



by swinging pendulums in them*, and by the torsional vibrations of an immersed diskf, 

 and of a sphere filled with the fluid J. 



The method of transpiration through tubes is very convenient, especially for compa- 

 rative measurements, and in the hands of Gkaham and Poiseuille it has given good 

 results, but the measurement of the diameter of the tube is difficult, and on account of 

 the smallness of the bore we cannot be certain that the action between the molecules of 

 the gas and those of the substance of the tube does not affect the result. The pendulum 

 method is capable of great accuracy, and I believe that experiments are in progress by 

 which its merits as a means of determining the properties of the resisting medium 

 vdll be tested. The method of swinging a disk in the fluid is simple and direct. 

 The chief difficulty is the determination of the motion of the fluid near the edge of the 

 disk, which introduces very serious mathematical difficulties into the calculation of the 

 result. The method with the sphere is free from the mathematical difficulty, but the 

 weight of a properly constructed spherical shell makes it unsuitable for experiments on 

 gases. 



In the experiments on the viscosity of air and other gases which I propose to describe, 

 I have employed the method of the torsional vibrations of disks, but instead of placing 

 them in an open space, I have placed them each between two parallel fixed disks at a 

 small but easily measurable distance, in which case, when the period of vibration is long, 

 the mathematical difficulties of determining the motion of the fluid are greatly reduced. 

 I have also used three disks instead of one, so that there are six surfaces exposed to 

 friction, which may be reduced to two by placing the three disks in contact, without 

 altering the weight of the whole or the time of vibration. The apparatus was constructed 

 by Mr. Becker, of Messrs. Elliott Brothers, Strand. 



Description of the ApjXJ^rafiis. 



Plate XXI. fig. 1 represents the vacuum apparatus one-eighth of the actual size. 

 M Q R S is a strong three-legged stool supporting the whole. The top (M M) is in the 

 form of a ring. E E is a brass plate supported by the ring M M. The under surface is 

 ground truly plane, the upper surface is strengthened by ribs cast in the same piece with 

 it. The suspension-tube A C is screwed into the plate E E, and is 4 feet in height. 

 The glass receiver N rests on a wooden ring P P with three projecting pieces which 

 rest on the three brackets Q Q, of which two only are seen. The upper surfaces of the 

 brackets and the under surfaces of the projections are so bevilled off, that by slightly 



* Bailt, Phil. Trans. 1832 ; Bessel, Berlin Acad. 1826 ; Dijbuat, Principes d'Hydratilique, 1786. All these 

 are discussed in Professor Stokes's paper " On the Effect of the Internal Friction of Fluids on the Motion of 

 Pendulums," Cambridge Phil. Trans, vol. ix. pt. 2 (1850). 



t Coulomb, Mem. de I'lnstitut national, iii. p. 246 ; 0. E. Meter, Pogg. Ann. cxiii. (1861) p. 55, and 

 CreUe's Journal, Bd. 59. 



t Helmholtz and Pietbowski, Sitzungsberichte der k. k. Akad. April 1860. 



