8 4 



NATURE 



\_Nov. 27, 1 1 



Now, since in Iceland spar the optic axis makes an 

 angle of very nearly 45 (strictly, 44° 36') with the natural 

 faces of the rhombohedron, all that is required is to obtain 

 an even cleavage-plate of the spar, about 2 cm. X 1 cm. 

 a id about 2 mm. thick, to break it in half, to turn one 

 of the pieces round in a plane parallel to its surfaces 

 through an angle of 180 from its position when broken 

 off, and to cement it on the other piece in this position 

 with Canada balsam or dammar. 



Then, on placing the combination in a polariscope (for 

 instance, laying it on the eye-lens of a microscope with 

 analyser just above it) the series of ellipses will be well 

 seen. Sodium light, e.g. that from a Bunsen burner with a 

 bead of sodium carbonate held in the flame, must be used. 



Prof. Ohm refers to a paper by Langberg (which I 

 have not been able to get a sight of) in which the occur- 

 rence and form of these bands were predicted from theory ; 

 so that the case resembles those of Airy's spirals and 

 Hamilton's conical refraction. 



A pair of plates with surfaces making an angle of 70° 

 (or more) with the optic axis also show these ellipses ; and 

 perhaps more instructively, since with such plates it is 

 easy to trace the origin of the bands in the coalescence 

 of portions of the circular isochromatic bands of high order 

 which surround the optic axis in each plate. 



Those who have a pair of Savart's plates mounted so 

 that one can rotate over the other, will find it most inter- 

 esting and instructive to watch (in monochromatic light) the 

 changes in form and character of the interference-bands 

 as the azimuth of one of the plates is gradually altered. 



Eton College H. G. Madan 



CONTINUOUS AUTOMATIC BRAKES 

 THE returns of the Railway Department of the Board 

 ■*■ of Trade serve as an excellent index to the defects 

 in the management and working of the railway system in 

 this country, the defects being brought to light during 

 the investigations of the trivial casualties and disastrous 

 accidents which take place, and inquired into by the 

 experienced inspectors of the Jioard of Trade. 



It is evident that by far the greater number of accidents 

 seem to have been caused by the trains not being fitted 

 with a really good brake, and in consequence being 

 unable to stop quickly in cases of emergency. Some 

 even have been caused by the brake itself failing to " go 

 on " when required, caused either by some defect in the 

 brake mechanism, or the design of the brake itself has 

 been bad, giving the engine-driver a false sense of security, 

 and leading the train with its living load into unnecessary 

 danger. 



It is a pity the railway companies do not pay more 

 attention to the conditions laid down by the Board of 

 Trade with regard to continuous brakes, stating the 

 qualities the brake ought to possess, for it is evident the 

 Board does not wish the adoption of any particular 

 patentee's brake, but a brake which includes to the 

 fullest extent the conditions laid down. It so happens 

 that the Westinghouse automatic brake answers the con- 

 ditions better than any other, and therefore the Board is 

 anxious to see it in general use, not because an ex- 

 inspector of the Board happens to be the chairman of the 

 English Westinghouse Brake Company, as the secretary 

 of one English railway seems to imagine, but because it 

 is the best brake. 



In an extract from the Board of Trade returns on 

 continuous bra':es for the half year ending June 30, 

 published by the Vacuum Brake Company, we find the 

 Westinghouse automatic credited with 397 faults for a 

 mileage of 15,506,447. 



We think it may be truly stated that the Westinghouse 

 automatic has not had fair play with some of the com- 

 panies having it partially in use, its failures having been 

 carefully reported, while any failure of their own special 



brake, not having any serious consequences, has been 

 looked over. 



Take for instance the returns sent in by the Midland 

 Company. Here the Westinghouse automatic has failed 

 thirty-seven times for a mileage of 374,390, or one fault 

 for every 10,1 iS miles, while the Midland automatic 

 vacuum has six failures reported for a mileage of 

 5,245,573, or one fault for every 874,262 miles run. ( n 

 the other hand we have the London, Brighton, and 

 South Coast Railway using the Westinghouse automatic 

 on the whole of their trains ; they report seventy-four 

 failures for a mileage of 3,122,510, or one fault for every 

 42,196 miles run. 



Why should the Westinghouse automatic run four 

 times as many miles per failure on the Brighton line than 

 on the Midland ? The reason is not far to seek ; on the 

 Brighton line the Westinghouse automatic is properly 

 looked after, and kept in good repair, while on the 

 Midland it has to stand back and give place to the 

 vacuum automatic, the Company's brake. 



The automatic vacuum brake in use on the Midland 

 Railway has, as its name implies, the pressure of the 

 atmosphere opposed to a partial vacuum for its motive 



power, the vacuum being created by means of an ejector 

 on the engine, connected to every vehicle on the train by 

 means of a continuous pipe, having flexible pipes and 

 couplings between the vehicles. To maintain the vacuum 

 throughout the train against leakage, there is a small 

 ejector continually in use on the engine. 



Coupled to the continuous pipe on each vehicle is the 

 automatic brake cylinder and reservoir peculiar to the 

 Midland automatic brake, the piston being connected by 

 means of levers and rods to the brake-blocks. The illus- 

 tration gives a good idea of the general construction of 

 the brake-cylinder and its connections, the arrangement 

 being as follows:— The brake-cylinder B is placed inside 

 the reservoir C, the piston A working air-tight in the 

 cylinder ; the piston-rod passing through the bottom of the 

 cylinder by means of a gland, e, having a flexible packing 

 ring, so arranged that when the piston is at the bottom of 

 the cylinder it comes in contact with the packing ring, 

 making an air-tight joint ; but when the piston moves 

 upwards, leaving the packing ring, air is able to pass 

 through the gland into the lower part of the brake-cylin- 

 der. The continuous pipe f is connected by the branch 

 pipe G to the lower part of the brake-cylinder. 



