produced  by  Rontgen  Bays  in  Different  Metals.        303 
take  the  curve  of  cooling  to  be  represented  by 
the  time  being  counted  from  the  moment  the  rays  are  cut  off. 
In  this  equation 
1  dY  ; 
and  one  may  get  rough  values  of  7  for  lead  and  for  zinc  by 
drawing  tangents  to  the  curves  of  cooling.  In  the  folio  wing- 
tables  are  several  values  found  for  different  points  of  the 
curves  in  fig.  3  ;  the  zero  for  the  zinc  curve  was  taken  as  the 
mean  of  the  position  at  2m  and  10m  and,  for  lead,  the  mean 
of  10m  and  18m. 
Zinc. 
Lead. 
y. 
1 
dY 
dt' 
7- 
V. 
cW 
dt- 
r- 
3-0 
1-8 
0-6 
6-0 
6-2 
1-0 
2-0 
1-6 
0-8 
3-0 
40 
1-3 
1-5 
1-05 
0-7 
1-5 
2-5 
]-7 
0'8 
0-50 
06 
0-8 
1-3 
1-6 
Average    
068 
Average   ...••- 
1-4 
In  order  to  determine  the  steady  value  from  the  value  at 
the  end  of  three  minutes  we  have 
V    = 
V 
,-yt 
which  gives,  for  lead  V^  =  13*95,  and  for  zinc  Vw  =  7*1. 
Approximately  the  same  rates  of  cooling  and  heating  were 
obtained  in  all  the  experiments  made  upon  this  point.  A 
second  example  is  given  in  fig.  4,  in  which  the  weaker  rays 
were  used  for  ten  minutes  instead  of  three.  Curve  I  is  for 
zinc,  curve  II  for  lead ;  the  two  experiments  were  made  at 
different  times  and  with  rays  of  different  intensity _,  so  that  the 
magnitudes  of  the  deflexion  have  nothing  to  do  with  each 
other.  To  show  that  the  inertia  and  damping  of  the  suspended 
system  have  no  sensible  effect  upon  the  determination  of  the 
rates  of  heating  and  cooling,  curve  III  is  added  for  comparison. 
