Meafurement of a Bafe on HouiiAow-Heath. 467 
to die proper pofition for the rod that is to be tried. Nevertheless 
the objeT lens remains in its original place; and in its dead 
another lens, of the fame focal didance, is fixed on a Similar 
end- piece,, that can be firmly clamped to any corresponding 
place whatever of the cheeks of the long Aide. Hence will 
appear the reafoti. for breaking the Screening tubes of the mi~ 
crofcopes into Several parts, and the ufe of the mahogany prifm, 
along which the thick part: of the tube moves from one end to 
the other. 
The pyrometer, fince it was firfl made and tried, has under- 
gone Several Small alterations, by way of improvements, which 
it is now unnecedary to defcribe particularly. One of thefe 
was the application of crofs levels to the parts of the tube (SS 
in the general plan) connected with the objecf glades. The 
manner in which they are fixed on will appear from the re- 
presentations of them in the lowermod left-hand angle of the 
plate. And the Section at the right-hand angle diews the ap- 
pearance of the double brafs hook, univerfal joint, and' milled- 
lieaded nut, applied, acrofs the middle of the boiler (at TU) 
whereby the levels are brought to be confident, when the water 
is boiling, with the pofition they had been adjufted to when 
the temperature was at freezing ; that is to Say, they are kept 
parallel to themSelves in both dates. This was thought necef- 
fary, becauSe the application of the boiling water funk the 
middle of the dide a Small matter, and thereby made the levels 
mn outwards. 
The micrometer So often mentioned, being a very edential 
part of the machine, is reprefented both in elevation and hori- 
zontal^ fedtion to the full fize.. Its chief parts confid of a mi- 
crometer deel Sc iew, which works in the fcpuare nut of a brafs 
Aide, while, the plane part of it enters into a long brafs focket. 
