﻿370 Messrs. J. H. T. Roberts and E. Meioh on the 



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The greatest difficulty met with in these experiments is 

 that the pressure necessary to produce flaring, or to bring a 

 jet to its condition of maximum sensitiveness, is very much 

 influenced by accidental irregularities in the orifice. Two 

 orifices, identical in dimensions and method of manufacture, 

 might be expected to give flames with identical properties, 

 but this is far from being the case. So important are small 

 irregularities in the rim, that an orifice which appeared 

 perfect when viewed through a powerful microscope would 

 produce variations of as much as 20 per cent, or 30 per cent. 

 in, say, the flaring-pressures of the jets from its two surfaces. 

 The quantities in the jet cannot be considered to be functions 

 of the measurable dimensions ol the orifice unless the jets 

 from opposite surfaces are identical. We have never suc- 

 ceeded in securing the complete fulfilment of the latter con- 

 dition, but the approximation depends upon the method of 

 manufacturing the orifices. After trying a variety of ways, 

 the method we found most successful was to grip the disk of 

 metal (about 0'15 mm. thickness) between the convex cheeks 

 of two curved pieces of hard brass strip, and to bore the hole 

 with a lathe twist-drill through all three. 



Various sources of sound were used to test the sensitiveness 

 of the flames, including bird-calls, whistles, pipes, ana* vibra- 

 ting bars. The sounds which affect these flames are all of 

 very high pitch ; as a general rule we found the sensitiveness 

 to increase with the diameter of the orifice, but in the case 

 of orifices above 2 mm. diam. the flames were very long and 

 somewhat inconvenient. On one occasion, with an orifice 

 1-107 mm. diam., we obtained a flame which, without the 

 aid of resonators, was practically as sensitive as the ear itself. 

 It responded to a whistle at the opposite end of the building, 

 and when the flame ceased to respond it was almost im- 

 possible to tell, by ear, whether the whistle was sounding or 

 not. 



Some results of observations on the jets from holes in thin 

 plates are given in the following table ; the minimum sensi- 

 tiveness pressure is an arbitrary quantity, depending upon 

 the pitch and intensity of the disturbing note. 



It will be seen from the table that there is a considerable 

 range of pressure over which the jet is sensifive. the deve- 

 lopment of sensitiveness with increase of pressure being very 

 gradual. It should be mentioned that jets from very small 

 holes in thin plates (about 0'3 mm. diam.) required about 

 80 cm. of water to make them flare, and did not exhibit any 

 sensitiveness. 



