114 Professor Forbes' s Researches on Heat. 



The differences for each grating, perhaps, do not exceed the 

 errors of experiment. In every case these numbers are infe- 

 rior to the geometrical interstices, but what inclines me to 

 think that this difference is due to the irregularities of figure 

 of the gauze (including the effect of flattening of the wires, 

 where they overlap, making the interstices obtuse-angled) is 

 this : that No. 2, in which the wires were finer compared to 

 the interstices than in others (the total interstices being one- 

 third part larger in proportion), and the gauze evidently far 

 more regularly formed than in the other cases, the percen- 

 tage transmitted differs very little from the geometrical gauge. 

 I own, at the same time, that a difference of 5 per cent, in No. 

 3 (which is evidently not due to an error of observation), seems 

 to me barely accounted for by this remark. 



34. Thread-Gratings. — With gratings of fine cotton-threads 

 T ^dth inch apart, used for showing Fraunhofer's Spectra, I 

 obtained a similar result. These threads were arranged paral- 

 lel-wise on two frames, capable of being superimposed rectan- 

 gularly. Thus, we can either employ a screen of parallel 

 threads one-hundredth of an inch apart, or a screen of mathe- 

 matically accurate squares, formed by superposition. It is 

 difficult in this case, however, to obtain the diameter of the 

 thread accurately enough to estimate the ratio of interstices. 



Per-centage of Incident Heat transmitted by Cotton-Thread 

 Gratings, T £(jdth inch apart. 



Thread Grating, Single 

 ______ Double 



Locatelli, with 

 Glass. 



29-5 

 90 



Dark Heat. 



30-2 



8*3 1 



* Corresponding single action = 30- per cent. 



t = 28-8 



The difference here seems imperceptible, the differences, such 

 as they are, being in opposite directions. The results in the 

 last column are from single experiments (November 28th, 

 1839). 



35. Action of Powders. — Adhering to the idea (12.) that 

 the action of a smoked surface was due to the mechanical ac- 

 tion of a number of minute opake points distributed over 

 a transparent body, it occurred to me almost at the com- 

 mencement of these experiments, to try the effect of powders 

 artificially sifted on such a surface. Any ingredient, how- 

 ever, which could make the powders adhere to the surface, 

 would hae vitiated the experiment, by introducing its own 



