Lt * “a, 
Further Remarks on Dr. Bradley’s Theorem, &'c. 23 
is wonderful,”’ says Professor Robison, “ that in a matter of such 
unquestionable importance the public has not enabled some 
persons of judgement to make proper trials ;” and we cannot 
without regret remark, that nothing worth notice has been added 
to our experimental knowledge since Robison wrote his excel- 
lent articles on the Strength of Materials. 
If the rule given above is correct, the following table will 
show the weight that would crush cylinders of different kinds of 
materials. 
Weight in pounds 
that will crush a 
cylinder whose 
base is one foot 
in area. 
Direct co- |\Veight in, pound: 
hesion of {that will crush a 
a square |cylinder an inch 
inch. in diameter. 
Material. 
lbs. 
| 50,000 314,160 57,600,000 
3,000 | 18,849 | 3,456,000 
| Cast iron ... 
edd (e000: 
Freestone. ..| 1,000 6,255 1,152,000 
Fine freestone 205 1,288 236,160 
Good brick..| 280 1,759 322,560 
VII. Further Remarks on Dr. Brapiey’s Theorem for com- 
puting the Astronomic Refraction. By T.S. Evans, LL.D. 
F.L.S. 
Tae attention of astronomers has lately been much occupied 
with the astronomic refraction; and perhaps no other subject 
is so worthy their notice at this moment, since it is the greatest 
obstacle of any, to improvements and accuracy in that science. 
Much has ‘lately been done towards an exact determination of 
its quantity at all altitudes above the horizon, by the use of the 
modern circles of Ramsden, Troughton, and other eminent in- 
strument-makers.. The great care with which these have been 
divided, and may be adjusted; the minuteness with which 
they may be read off, by means of micrometers, placed in the 
microscopes; and the certainty with which the object can be 
bisected, in the field of view, by using cobwebs, or perhaps the 
minute platina wires lately made by the ingenious Dr. Wollaston, 
give us great reason to hope, that with the zeal of our present 
best observers much time will not elapse, before the difficulties 
now met with in discovering the quantity of the refraction, will 
in a great measure, if not wholly, be removed. 
Of all the different formule that have hitherto been published, 
Ba4 it 
