394 Progress of Invention. 



instrument of this description. It is founded on the fact that the 

 images of two objects, the rays from which traverse portions of air 

 having different degrees of transparency, will be of different 

 brightness ; and that, if their brightness is equalized, the amount 

 of adjustment necessary for the purpose will be a measure of the 

 difference of the transparency of the two portions of air through 

 which the rays from them respectively have passed. To secure 

 accuracy of result, the two objects must be seen with the same eye, 

 in the same general direction, and in the same conditions ; and the 

 light from other objects must be excluded. M. De La Hive's in- 

 strument consists of two tubes, having each an objective at one end, 

 and their other ends attached to a common eye-glass, of which each 

 objective takes up half the field. The optical axes of the two ob- 

 jectives form an angle which may vary from 0° to 29°, at the will of 

 the observer. The rays passing along the principal axis of each 

 objective are made parallel with the axis of the eye-glass by two 

 rotal and successive reflections — the first from a moveable and the 

 second from a fixed prism. The movement of the moveable prism 

 is so connected with that of the moveable tube, that the angle de- 

 scribed by the prism is half that described by the tube. Whatever 

 the points towards which the tubes are directed, the images of these 

 points are in juxtaposition in the focus of the eye-glass. The in- 

 strument is proved to be properly adjusted by turning it round 

 through 180°, so that the objects are seen the first and second times 

 through different tubes. The images obtained are equalized by the 

 means used with ordinary photometers. This instrument will 

 measure the comparative brightness of two stars, or of different 

 portions of the heavens. 



New Gauge for Steam Boilers. — One of the most frequent 

 causes of steam-boiler explosions, is an insufficient water supply. 

 This arises in some instances from neglect on the part of those in 

 charge, but more usually from the difficulty of ascertaining the 

 water level within the boiler. Many modes of automatic indication 

 of a deficiency of water have been employed, and to a greater or less 

 extent with advantage, but none are sufficiently effective to remove 

 any possibility of accident. An American has, however, devised 

 an apparatus for the purpose which is very simple, and appears to 

 be very reliable. It consists in a tube fixed into the boiler a little 

 below the proper water level, and projecting for some distance out- 

 wards, the internal extremity being open, and the outward closed. 

 Around this tube is a casing, and the annular space between it and 

 the tube is filled with water. As long as a proper water level is main- 

 tained in the boiler, the tube remains full of the fluid, but as soon as 

 the water in the boiler falls below the opening of the tube, the latter 

 becomes filled with steam or foam, and the water in the annular 

 space around it boiling, steam is generated, and passes into a space 

 prepared for it, where it blows a whistle, and even, if desirable, acts 

 on a lever that opens the safety valve of the boiler and allows the 

 escape of steam. Thus, not only is there notice of danger, but it 

 is considerably diminished, until the proper measures are taken. 



Undulating Railways. — The idea of an undulating railway is 



