Prof. Challis on a Theory of Mayietie Force. 65 
Depth of the Corresp. Value of v 
column of |Corresp. time velocity by formula 
liquid, = a of ascent, rig: 
h. , v. = 8°69. 
1 
6 0 | tease re 
OnE Se Seman 1 L | 
5 1:58 ae ates A 
AL Pere | 
Be ssanine 1°93 1°93 
ed 3°51 L oe 
ns oS 2°4 2°48 
3 5°91 ‘ : 
{ oe a os ae 3°35 3°49 
Ae 9-26 
IL wes a 
= Nd RQR eee 5°65 579 
1 14-91 | ‘ ‘ 
a aes 17°84 17°38 
| 0 32°75 
The near coincidence of the results in the third and fourth 
columns shows that in this case the rate of filtration varies directly 
as the pressure upon the filter. 
With a double filter the rate of discharge was found to be 
reduced to one-half very nearly ; and with a filter of =2;ths of an 
inch diameter, that is, one-fourth of the surface of the filter in 
the above experiment, the rate of discharge was found to be 
reduced to one-fourth of that of the above experiment. Hence 
we conclude that the rate of filtration varies directly as the areas 
of the surface of the filter in contact with the liquid, and iversely 
as the thickness. 
[To be continued. | 
VIll. A Theory of Magnetic Force. 
By Professor CuHauiis, F.R.S* 
ps my last communication, containing a theory of galvanism, 
no allusion was made to thermo-electric phenomena. This 
omission I propose to supply before proceeding to the theory of 
magnetism, the fact that galvanic currents may be produced by 
heat, being confirmatory of the explanation given by hydro- 
dynamics of the generation of secondary currents generally, 
whether electric, or galvanic, or magnetic. 
1. It isassumed in that explanation that in the neighbourhood 
of the earth’s surface there are steady etherial currents, which 
eventually may be found to be secondary with respect to other 
more general currents, but for considerable spaces may be con- 
sidered to be uniform as to velocity and density. These currents 
are supposed to flow freely through the interior of bodies, with 
only such modifications as may result from the arrangement of 
* Communicated by the Author. 
“Phil, Mag. S, 4. Vol. 21, No, 137, Jan. 1861. F 
