85 
Differentiate with respect to x, supposing t an arbitrary 
constant, 
1 to v dtj _l_x 
_ dv dz x 2/ct 
dt 
which equated to 0 gives Mr. Darwin’s relation x 2 = 2ht. 
But if you assume x to be an arbitrary constant and diffe- 
rentiate with respect to t, you get 
_l_. cflog (~§)_ 3 * 
civ dt 2(5 4 /ct 2 
dt 
• OC ^ 1 
which equated to 0 gives t = • 
These two relations between x and t show that in any plane 
the maximum rate of cooling takes place in one third of the 
time (measured from the commencement of the cooling) that 
the maximum rate of cooling of the mass takes place in that 
plane. 
The conclusion from this appears to be that if it be ad- 
mitted that the contraction is proportionate to the cooling, 
the principal contraction in any plane will take place long 
before that place is the seat of the maximum cooling of the 
mass. 
I ought to have stated above that both x 2 = 2 kt and x 2 = QJct 
give negative values for the second differential coefficient. 
That is, of course they do give maxima and not minima. 
“ Arrangement of a Tangent Galvanometer for lecture-room 
purposes to illustrate the laws of the action of currents on 
magnets, and of the resistance of wires,” by J. H. Poynting, 
B.A., Demonstrator in the Physical Laboratory of the Owens 
College. 
Three coils of similar wire are arranged round the circum- 
ference of a circle with a compass needle at the centre, each 
wire going only once round the circle. The six ends of the 
