proposed to be erected at Montrose. 281 
weight amounts to at least 500 or 1000 tons on each end of the bridge, this 
being the greatest weight the bridge itself will have to sustain. 
Strength The bridge will carry, with perfect safety, 7000 people in addition 
to its own weight ; and with this great load, no part of the chains will be 
stretched with a greater strain than 8 tons on the square inch, which it is 
well known common iron will bear with the utmost safety, but which, at 
any rate, every bar will be proved to before it goes into the bridge. 
Expence — Estimated at £12,648. 
Art. VIII. — Experiments and Observations on Radiant Heat. 
By William Ritchie, A. M., Rector of the Academy at 
Tain. 
1. .Having procured a differential thermometer with large 
bulbs, blown very thin, and quite transparent, I placed a heat- 
ed ball exactly in the middle between the bulbs, and observed 
that the fluid in the stem remained stationary. I then coated 
the exterior hemisphere of one of the bulbs with lamp-black, 
and placed the heated body in the same situation as formerly. 
The instant the ball was placed in its proper position, the fluid 
in the stem of the blackened bulb began to descend, and the 
number of degrees which the fluid descended increased as the 
temperature of the body was raised. 
This experiment appears to me completely decisive of the 
fact, discovered by De Laroche, that invisible caloric permeates 
thin plates of glass in the same manner as light. For it must 
either pass through glass in the same manner as light, or be 
conducted through it as if the glass were an opaque substance. 
Now, upon the latter supposition, the only effective part of the 
heat upon the included air, would be that portion of it which 
was absorbed by the interior hemispheres of the bulbs, and gra- 
dually conducted to the included air; and as these hemispheres 
are equal, the quantity of caloric absorbed by each will also be 
equal. Hence it is obvious that the fluid in the stem must re- 
main stationary till the heat reach the exterior hemisphere of 
each of the bulbs. Now, as this would require some time (since 
glass is a bad conductor of caloric), how are we to account for 
the instantaneous depression of the fluid in the stem ? Again, 
since lamp-black allows caloric to radiate more copiously than 
