Meafurement of a Eafe on Hounflow-Heath. 467- 



tb the proper pofition for the rod that is to be tried. Neverthelefs 

 the object lens remains in its original place; and in its flead 

 another lens^- of the fame focal diftance,. is fixed on a fimilar 

 end- piece,, that can be firmly clamped to any correfponding 

 place whatever of the cheeks of the long Aide. Hence will 

 appear the reafon for breaking the fcreening tubes of the mi- 

 crofcopes into feveral 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 feveral fmall alterations, by way of improvements, which 

 it is now unneceflary to defcribe particularly. One of thefe 

 was the application of crofs levels to the parts of the tube (SS 

 In the general plan) conne6led wdth the object glafles. The 

 manner in which they are fixed on will appear from the re- 

 prefentations of them in the lowermoft left-hand angle of. the 

 plate. And the fedion at the right-hand angle fhews the ap- 

 pearance .of the double brafs hook, univerfal joint, and milled- 

 headed nut, applied . acrofs the middle of the boiler (at TU) ? 

 whereby the levels are brought' to be confiflent, when the water 

 is boiling, with the polition they had been adjufted to when • 

 the temperature w^as at freezing; that is to fay, they are kept - 

 parallel to themfelves in both ftates. This was thought necef*- 

 fary, becaufe the application of the boiling water funk the 

 middle of the ilide a fmall'matter, and "thereby made the levels 

 run outwards. 



The micrometer fo often mentioned, being a very eiTential 

 part of the machine, is reprefented both in elevation and horir 

 zontal fedlion to the full fize. Its chief parts confifl: of a ^mi- 

 crometer fteelfcrew, which works in the fqu are nut of a brafs 

 Hide, while the plane part of it enters into/ a long brafs focket, 



nicely 



