834 Report of the Committee 
There is another circumftance that we have not yet 
taken notice of, which, in ftridtnefs, caufes fome error 
in thermometers, namely, the difference of expan lion of 
the glafs tube and the fcale. But this error is in almoft 
all cafes fo fmall as to be not worth regarding ; we have, 
however, in the note below given a rule for computing 
the value of it ls K 
(g) The ufual way of adjutting thermometers is, to mark the boiling and 
freezing points on the glafs tube, and not to fet off thofe points on the fcale till 
fome time after, when the tube and fcale may both be fuppofed to be nearly of 
the temper of the air in the room; confequently, when the thermometer is 
' expofed to a greater heat than that, the fcale, if of brafs, will expand more 
than the glafs tube, and the divifionson it will be longer than they ought to be; 
but, if the fcale be of wood, it will expand lefs than the glafs tube, and the 
divifions will be too fhort. Let now the heat of the air, when the divifions 
were fet off on the fcale, be called a; let the degree of heat which the ther- 
mometer ttands at in the experiment be called d; and let the degree anfwering 
to that point of the fcale in which the thermometer is fattened to the fcale be 
called F. Then, if all parts of the thermometer and fcale are heated equally, 
and the fcale is of brafs, the thermometer will appear to ttand lower than it 
ought to do by the part of a degree, obferving, that if d —f x d — a 
is negative, it will ttand higher than it ought to do; but if the fcale is of wood, 
it will ttand higher than it ought to do by the part of a degree. 
If the thermometer be fattened to the fcale by the ball, or any part of the tube 
lower than the obferved heat, the error will be the fame, whether that part of 
the tube and fcale, which is above the obferved degree, be of the lame heat as 
the ball or not : but if the thermometer is fattened to the fcale by the top of the 
tube, as is frequently done, then the error will vanilh whenever that part of 
the tube and fcale, which is above the obferved degree, is not much heated. 
This rule is founded on Mr. smeaton’s experiments, who found, that wdiite 
glafs expands x^oth of an inch in a foot by 180° of heat; that brafs wire 
expands xlMoj and that wood expands fcarce fenlibly. 
la 
