STANDARD  THERMOMETERS. 
255 
STANDARD  THERMOMETERS. 
To  tbe  Editor  of  the  Chemical  News  : 
Sir, — As  the  question  touched  upon  by  your  correspondent, 
"  Zero,"  about  standard  thermometers  is  of  considerable  import- 
ance on  account  of  the  comparison  of  accurate  thermometric  ob- 
servations— as,  for  example,  those  suggested  by  Drs.  Compton 
and  Aitkins  for  the  careful  observations  of  the  temperature  of 
the  human  body  during  disease — I  venture  a  few  words  on  the 
point. 
In  your  article  it  is  stated  that  Mr.  H.  C.  Kay's  instrument, 
as  certified  by  Mr.  Glaisher,  differs  1J°  from  the  instrument 
with  a  Kew  certificate.  This  statement  implies  at  once  that 
there  are  either  two  standards  in  England,  that  of  Kew  and  that 
of  Greenwich,  differing  1J°  from  each  other,  or  that  Mr.  Glaisher 
must  have  made  a  mistake  of  1  J°  by  the  comparison  of  this  in- 
strument. Now  neither  of  these  suppositions  is  probable,  for  it 
is  not  likely  that  the  standards  at  Kew  and  Greenwich  should 
differ  to  any  measurable  unknown  amount,  nor  is  it  likely  that  a 
man  like  Mr.  Glaisher  would  make  such  a  mistake.  It  is,  there- 
fore, evident  that  we  must  otherwise  account  for  this  difference. 
I  applied  to  Mr.  F.  Pastorelli,  of  208  Piccadilly,  one  of  the  most 
careful  and  accurate  manufacturers,  for  an  explanation,  which  he 
was  kind  enough  to  give  me  at  once.  Mr.  Pastorelli  informed 
me  that  it  was  a  fact,  known  to  most  practical  and  accurate 
manufacturers,  that  thermometers  made  in  the  usual  way  did 
acquire  gradually  an  increasing  index  error  which  would  reach 
the  maximum  of  1  to  2  degrees  about  a  year  after  the  manufac- 
ture, but  remain  then  as  good  as  constant.  Mr.  Pastorelli  be- 
lieves this  increasing  error  to  be  owing  to  the  contraction  of  the 
glass  after  its  manufacture.  He  explained  to  me  a  process  he 
followed  to  avoid  this  after-contraction  as  much  as  possible;  but 
this  I  shall  pass  over  as  being  merely  important  to  manufac- 
turers. 
After  the  above  explanation  it  is,  therefore,  very  probable 
that  Mr.  Kay's  instrument  acquired  this  greater  additional  index 
error  first  after  its  comparison  by  Mr.  Glaisher  with  the  standard. 
But  how  have  we  to  account  for  it  that  the  instruments  with  the 
