638 
ME. AIET’S ACCOUl^T OE THE COXSTEUCTIOX OE 
flexure in altering a measure cut upon the upper surface would be much less for a thin 
scale than for a thick one. In order to test this, Mr. Baily caused two shght marks to 
be cut upon the stout bar of the Royal Society’s scale, and made fire experiments on 
1843, December 24, of which the following numbers are the means. The wmes used 
were common large pins. 
Measures of intervals upon I 
the slender scale | 
Bar laid flat 
Wire under the middle 
Wires under the ends 
div. 
0-0 
+ 0-1 
- 6-0 
Measures of intervals upon J 
the surface of the stout bar 
Bar laid flat 
Wire under the middle 
Wires under the ends 
0-0 
+ 5-2 
— 42*0 
In order to collect together the observations which may be considered as apphung to 
the reduction of comparisons of standards, before entering upon the comparisons them- 
selves, I shall interrupt the chronological order (as I have akeady done) to give the 
results of experiments on thermometric expansion made by ISIr. Simms, at 138 Fleet 
Street, under the general instruction of Mr. Baily, -with some suggestions by myself. 
A tin trough was prepared, 44 inches long, 5 inches wide, and 5 inches deep, supported 
on four screws. Glass windows were provided in proper places for admitting lateral illu- 
mination. A lever-frame with eight rollers (on the principles to which I have akeady 
adverted) was placed in the trough, and the bar which was the subject of experiment 
was placed upon the rollers. The trough was filled with water, of any requked tempe- 
rature, which could be heated by spirit-lamps below the trough. The microscopes were 
fixed to a stone ledge built into the wall of the room. The object-glasses of the micro- 
scopes (which were common object-glasses of unusually short focal length) were im- 
mersed in the water. At all ordinary temperatures the Gsion was quite distinct, even 
with the trough uncovered : when the temperature Avas high, the Aision was mdistinct. 
except the trough was covered, but was very good AA^hen it Avas covered. The thermo- 
meters were placed in the water, on the surface of the bar. I imagme that these expe- 
riments may challenge comparison with any that ever Avere made, m every respect except 
one, viz. that the thermometers employed, though probably as good as any then m exist- 
ence, did not possess the authority which was acquked by thermometers subsequently 
made by independent graduation. 
A screw-apparatus was provided, acting in opposition to a sprmg, by which the line 
at one end of the bar could be brought under the microscope at that end ; and the 
micrometer-movement of that microscope was never used. The value of one dmsion of 
the micrometer which was employed was ascertained to be 0-000025287 inch. The bars 
which were the subject of examination Avere eAudently not the same as those already 
mentioned; for the gun-metal bars are all described at this time (1844, Februai-j^) as 
being 40 inches long; whereas, of the formerly-mentioned bars. No. 4 had been broken 
and No. 3 had been shortened before the end of 1843. 
