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it appeared full of large cavities on the infide, though 
the outfide feemed as fair as any other. Some of 
thefe cavities were above an inch long, and the fur- 
rounding metal, after filing, not above the thickneis 
of a card. I would therefore recommend the weigh- 
ing bars of this metal, in order to obferve the dif- 
ference of expanfion. 
The other bars of this inftrument, as the founder 
informs me, are made of 1 8 parts fpelter, and 2 of 
copper. I fhould imagine, that there is a greater 
proportion of copper than he owns, from the ap- 
pearance of the metal, and the difproportion in its 
expanfion, to what I fuppofe 2 parts in 20 might 
occafion. 
Since this inftrument has been made, I found, on 
looking over the 1 oth volume of Martin’s Abridgment 
of the Philofophical Tranladtions j that Dr. Mortimer 
had, in 173 5, given the Royal Society a defcription 
and drawing of an inftrument he invented for the 
purpofe j and that Mr. Johnfon had alio given a 
drawing of another, invented by Mr. Fothringham. 
Although thefe are of a very different conftrudion, 
yet, as they were formed on the fame principles, I 
ought, in juftice to thefe gentlemen, to mention 
them. 
I have made what obfervations I could on this in- 
ftrument, fince the Ihort time it is made ; by com- 
paring it with a Fahrenheit’s, and a fpirit thermo- 
meter ; and find, that it keeps at a medium between 
both ; not riling at firft fo quick as the mercury, and 
fomewhat quicker than the fpirit. Qn placing them 
together in the fun, when its heat became intenfe, 
it rofe at laft falter than the mercury, and not fo faff 
as 
