344 Analysis of Books. 



idea might be formed of the extent by the acuteness or gravity of the 

 sound produced, the edges of the concavity serving- as bridges 

 which limited the length of the string. So delicate is this test, that 

 a concavity can be detected by this method, when the interval be- 

 tween the wire and the surface under examination is imperceptible 

 to the eye. 



The error in question, resulting from the extension and compres- 

 sion of the surfaces of the bar dependant upon its curvature is 

 obviated in the following manner: The neutral surface which 

 suffers neither extension nor compression is shewn by the author to 

 be at about one- third of the thickness of the bar from the surface 

 which becomes convex. When the object is to have two points only 

 on the bar, by cutting away one-half of the thickness of the bar at 

 its ends, and placing the points upon the new surfaces, the error 

 is reduced to the least possible quantity. But when a scale of inches 

 is required, the nearest approximation to correct measurement is 

 obtained by diminishing, as much as possible, the thickness of the 

 bar. and by providing another bar on which it is to be supported, 

 and on which it is allowed to slide freely in a dovetailed groove 

 formed by two side plates of similar thickness, screwed to the sur- 

 face of the bar, and to which it is to be fixed at its middle point by 

 a single screw passing through it. 



8. On the Illumination of Light-houses. By Lieut. Thomas Drum- 

 mond, of the Royal Engineers. 



THE author, after briefly describing the different methods at present 

 employed for illuminating light-houses, proceeds to detail what he 

 considers an improvement upon those now in use. This consists in 

 substituting for the Argand burners a small ball of lime, ignited by 

 the combustion of oxygen and hydrogen. 



From this small ball, only three-eighths of an inch in diameter, so 

 brilliant a light is emitted, that it equals in quantity about thirteen 

 Argand lamps, or 120 wax candles; while, in intensity or intrinsic 

 brightness, it cannot be less than 260 times that of an Argand lamp. 

 These remarkable results are deduced from a series of experiments 

 made lately at the Trinity-house ; and, having been repeated with 

 every precaution, and by different individuals, there seems no reason 

 to doubt their accuracy. In the best of our revolving lights, such 

 as that of Beachy Head, there are no less than thirty reflectors, ten on 

 each side. If, then, a single reflector, illuminated by a lime ball, be 

 substituted for each of these ten, the effect of the three would be 

 twenty-six times greater than that of the thirty. On account of the 

 smaller divergence of the former it would be necessary to double 

 their number, placing them in a hexagon instead of a triangle. In 

 this case the expense is estimated at nearly the same. This method 

 was tried lately at Purfleet in a temporary light-house, erected for 

 the purpose of experiments by the corporation of the Trinity-house, 

 and its superiority over all the other lights with which it was con- 

 trasted was fully ascertained and acknowledged. 



