311 



the other coated inside with a thin film of pounded charcoal. The 

 latter was most affected. 



In a third experiment, a frame of glass threads, or fine wire, placed 

 vertically, was coated with a film of diluted white of egg, applied 

 with a broad hair brush, and kept constantly at the same tempera- 

 ture by applying it fresh and fresh. Then, exposing a heated ball 

 on one side, and an air-thermometer on the other, no effect was sen- 

 sible on the latter, when the temperature of the ball was low, but 

 when just invisible in the dark the effect was very sensible. 



The author also finds that this effect is greater than in the case of 

 glass, and that liquid screens are more permeable to heat than solid 

 ones. He also found that little difference of effect is observed whether 

 the screens be near to or far from the heated ball, cceteris paribus ; 

 and this he considers as demonstrating that the effect was not due 

 to secondary radiation from the screen. 



On the Derangements of certain Transit Instruments by the effects of 

 Temperature. By Robert Woodhouse, A.M. F.R.S. %c. Read 

 April 26, 1827. [Phil. Trans. 1827, p. 144.] , 



In the Philosophical Transactions for 1825, the author alluded to 

 the derangement of the Cambridge transit instrument, arising from 

 unequal expansion of its braces, establishing, as he conceived, the 

 fact and cause of such derangement ; and in a subsequent paper in- 

 stanced its effect in one case as altering, by no less than 20 ', the 

 time of the passage of the pole star over the wires. In consequence 

 the removal of the braces was resolved on, but from one cause or 

 other delayed ; but the author considers good to have arisen from 

 this procrastination, as enabling him to make further experiments, 

 which he was led to do in consequence of Mr. South's observations, 

 which lead to conclusions opposite to those deduced by himself. To 

 satisfy his own mind, therefore, he instituted the series of experi- 

 ments described in this paper. 



His first care was to determine precisely, by a series of transits, 

 the polar intervals between the wires of his eye-piece. He then ob- 

 served the pole star at its lower culmination, and after its passage 

 over the middle wire, applied a warm blanket to the upper eastern 

 and lower western brace, and found that a deviation of the telescope 

 to the west had taken place, such as to alter the passage over the 

 remaining wires nearly 19 seconds, and in the direction corresponding 

 to the expansion of the braces. Another observation, under more 

 favourable circumstances, gave a similar result; viz. 18 seconds of 

 retardation. 



In another experiment the warm blankets were applied to the 

 upper western and lower eastern braces, when deviations appeared 

 to have taken place to the extent of 36, 29, and 27 seconds in the 

 respective passages over the 5th, 6th, and 7th wires. 



In another trial the passages over the three first wires were ob- 



