114 



KNOWLEDGE 



[Jl'LV H, 



o'nJuctor. should bo scfficii^ntly protcotcil witli suitalilo safety 

 fasps. so that no portion of the conductor should ever be allowed to 

 attain a tempcrntnre ciceedinp 150 dog. F. 



X.B. — These fuses ore of the very essence of safety. They should 

 aln-ays be enclosed in ineonibustiblo cases. Even if wires become 

 perceptibly warmed by the oniinnry current, it is a proof that they 

 arc too small for the work they have to do, and that they ought to 

 be replaced by larger wires. 



10. Under ordinary circumstances complete metallio circuits 

 should be used, and the employment of gns or water pipes as con- 

 ductors for the purivose of completing the circuit should in no case 

 be allowed. 



11. Where bare wire out of doors rests on insniatiug supports it 

 should be coated with insulating mntoritil, such as intlia-rubbor, tape, 

 or tube, for at least two feet on each side of the support. 



12. Bare wires passing over the tops of houses should never bo 

 less than 7 ft. clear of any )iart of the roof, and they should in- 

 variably be high enough, when crossing thoroughfares, to allow firo 

 escapes to pass under them. 



13. It is most essential that the joints should bo electrically and 

 mechanically perfect. One of the best joints is that which is 

 whip[>e<l around with small wire and tho whole mechanically united 

 by solder. 



14. The position of wires when underground should be cfTiciently 

 indicated, and they should be laid down so as to be easily inspected 

 and repaired. 



15. AH wires used fur indoor purposes should be efficiently 

 insulatol. 



!G. When these wires pass through roofs, floors, walls, or par- 

 titions, or where they cross or are liable to touch metallic masses 

 like iron girders or pipes, they should be thoroughly protected from 

 abrasion with each other or with tho metallic raa»ses, by suitable 

 additional covering ; and where they are liable to abrasion from 

 any cause, or to the depredations of rats or mice, they should bo 

 efficiently enca«ed in some hard material. 



17. Where wires are put out of sight, as beneath flooring, they 

 should be thoroughly protected from mechanical injury, and their 

 position should be indicated. 



X.U. — The value of frequently testing the wires cannot be too 

 strongly urged. It is an operation, skill in which is easily acquired 

 and applied. The escape of electricity cannot bo detected by tlio 

 sense of smell as can gas, but it can be detected by apparatus f.ir 

 mere certain and delicate. Leakage not only means waste, but in 

 the presence of moisture it means destruction of the conductor and 

 its insulating covering by electric action. 



111. Lamp-s. 

 18. Arc lamps should always bo guarded by proper lantt 



to 



prevent danger from falling incandescent pieces of carbon, and from 

 ascending sparks. Their globes should be protected with wire 

 netting. 



19. The lanterns and all paits which are to be handled should bo 

 insulated from the circuit. 



IV. Da.nges to Person. 



20. To secure persons from danger inside buildings it is essential 

 BO to arrange the conductors and fittings that no one can be exposed 

 to the shocks of alternating cnrrents exceeding CO volts ; and that 

 there should never bo a difference of potential of more than 200 volts 

 between any two points in the same room. 



21. If tho difference of potential within any house exceeds 200 

 volts, whether tho source of electricity bo external or internal, the 

 house should be provided outside with a " switch," so arranged that 

 the tupply of electricity can be at once cut off. 



By order of the Council, 



F. H. Wkbd, Secretary. 

 Offices of the Society, 4, The Sanctuary, Westminster, 



A THEORY OF FORESIGHTS. 



TUO.SE of our readers who tako any interest in rifle-shooting 

 know that the »mall-l«,ro Match riflo is fitted with a wind- 

 Kan'-'P, "n which is fcrewcd a foresight suitcfl to the vision of tho 

 marknman. These sights vary acconling to fancy, those most in 

 use biing the calliper, the solid bead, and the hollow bead or ring. 

 By m<-ans of tho wind-gauge, it will Ixj understood that tho bull's 

 eye may always be in alignment with the sights. 



I pr<>|>o<ic to call attention to certain optical advantages attending 

 the noc of the hollow bead which, I believe, arc not generally taken 

 advantage of. 



The ballseye at tho long ranges (800, 900, and 1,000 j-ards) is 

 3 feet in diameter, and tho rings arc made of different sizes. If 



wo tako one -1 ind 

 eye being say -1 foi 

 follows : — 



diameter and ufo it at all three ranges, the 

 from the foresight, tho effect is roughly as 



m ® ® 



And it is obvious that, to obtain the perfect alignment necessary, 

 the bull's-eye must bo geometrically centred in tlio ring. As 

 wo get to tho longer ranges, tho bull's-eye fitting Fnialler, thin 

 geometrical centering becomes more ditlicult, a very slight error at 

 1,000 or 1,100 yards, in all probability, causing tho shot to miss 

 the bull ; we should, therefore, use a smaller ring as the range 

 increased. 



In the use of rings of different sizes I have made the following 

 observation : — 



Let us tako tho range at 800 yards, the bull's-eye 3 feet in 

 diameter, tho distance of the eye from the foresight (in the back 

 position with tho back-sight on the heel-plate) 4 feet: tho apparent 

 size of tho bull at tho end of the barrel will bo found to be OU inch. 

 Now, if we take a ring 'OG inch, tho effect produced is this, 



a portion of the black being hidden by tho diffraction fringe causecf 

 by light passing through the small ajjerture ; it is inexpedient there- 

 fore, to use a ring the exact size of tho bull, on account of tho 

 impossibility of detecting small errors. Although tho measurement 

 of diffraction fringe involves a somewhat e.\tcnsivo use of intogrnla 

 and differentials, it may be practically measured in this instance, 

 and becomes of very great importance. Let us tako a ling double 

 the apparent size, that is '12 inch. (I may here state that guumakorg 

 make these foresight rings accurately to tho hundredth of an inch, 

 and that Mr. Gibbs of Bristol has made mo several for experimental 

 purposes to thousandths ; for all practical purposes, however, 

 hundredths will be found sufficient.) 



The effect is this (tho diagi-am being drawn accurately) : — 



The diffraction fringe has all the appearance of, and nets in the 

 same manner as, a solid ring, and, holding the bull exactly, the 

 effect of a slight vibration or inaccuracy is this : — 



By using such a ring tho trouble of geometrically centering tlic- 

 bnll is got rid of, and wo may centre it optically; when tho bull is 

 seen jicrfectly it is centred, and the alignment of the sights ia 

 accurate. This centering cannot be mistaken, for the bull suddenly 

 seems to expand to a great size. 



(Jn this theory, and with the data given above, the rings for the 

 different ranges will be as follows : — 



Jiije 4 feet from Foreiijhl. 



800 yards 12 



900 „ -100 or roughly -11 or -10 



(tho smaller ring, I think, is iirjfurablo.) 



1,000 yards 'OilO or roughly -10 or •0!> 



1,100 „ -087 „ -09 



Eyeafl.Gin.fromFored'jId. 

 SCO yards -10.) or roughly 10 



