December 14. 1899 



NATURE 



153 



difficult to write down the values of the terms «, a, b^ 

 of the following expression from mere inspection of the 

 gear. In link motions it is more difficult at present, but 

 we are already seeing our way to easy rules. Here, 

 then, is the problem which Mr. Harrison has solved : — 



Given the ratio of length of connecting rod to that of 

 the crank. Given that the distance of the valve to the 

 right of its mid stroke (Fig. i) is 



y = c + a sin (e + a) + d sin (20 + 0), 



b being small in comparison with rt, show on a diagram 

 the position of the piston and the value of y when 6 has 



For any value of 6 : Make AOK = 6, project K verti- 

 cally to P ; p shows the position of the piston, ep is its 

 distance from the beginning of its stroke. OK cuts the 

 valve circle in Q. The perpendicular distance QN of g 

 from dCiD' is/, and the part of it QM is the opening of 

 the left-hand port to steam. Similarly, in the out stroke^ 

 when the crank has passed through the angle aok' which 

 is greater than i8o° ; project k' to p' to get the piston 

 position in the out stroke. Let OK' cut the valve circle 

 in q' ; then the perpendicular distance q'n' is the distance 

 of the valve to the left of its mid position (Fig. 2), and 

 q'm' is the opening of the right-hand port to steam. 



Fig. I. — A is called the inner dead point. 

 a' ,, outer ,, ,, 



The motion from a to a' is called the in stroke. 

 ,, ,, a to a ,, out ,, 



Fig. 2. — Valve shown in the middle of its stroke- 

 X Y and x' y' are the laps (sometimes called the 



steatn laps). 

 z w and z' w' are the inside laps (sometimes- 

 called the exhaust laps). 



any value. Further, the laps XY and xV (Fig. 2) being 

 given, show on the diagram the amounts of opening of 

 the ports to steam, these being obtained by subtracting 

 the laps from y or -y. 



With centre C (Fig. 3) and radius CA or Ca' representing 

 the crank, describe the crank circle aba'b'. Draw bcb' at 

 right angles to aca'. With centre on ca produced, and 

 radius equal to length of connecting rod, describe the 

 arc bob'. Make angle coCi = 0-a and 0Ci = 2^. We 

 give the name " false centre " (relatively to both circles) 

 to the point o. 



Piston nio\/inc av/ay from crank. 

 OUT STROKE. 



Fig. 3. — Mr. Harri.son's diagram showing positions of piston and valve 

 when crank makes an angle 8 with inner deao point. The valve dis- 

 placement to right of mid position (Fig. 2) being y, where 

 ^ = c + a sin (e -f o) -f * sin (2 e + /3). 



WMth centre Cj and radius a, describe the circle dqd'q'- 

 Draw dCjD' making an angle with aa' equal to a. Draw 

 ST parallel to dd' at a perpendicular distance from it 

 equal to the lap XV of Fig. 2. Draw s't' also parallel to 

 d'd at a perpendicular distance from it equal to the lap 

 x'v' of Fig. 2. 



Draw tangents at a, h, a', r.'. In the z« stroke of the 

 piston, when the crank moves from A to B to a', let us 

 show on EF the positions of the piston, and in the out 

 stroke, when the crank moves from a' to b' to A, let us 

 show on e'f' the positions of the piston. 



NO. 1572, VOL. 61] 



It is easy to see how we get the openings of the ports 

 to exhaust in exactly the same way. Lines joining a 

 with S, t, s', t' show the angular positions of the crank 

 when admission and cut off take place. In fact, we see 

 that this diagram gives us the positions of the piston 

 when admission, cut off, release and compression occur 

 both in the out and tn strokes. It gives us an easy way 

 to study how changes in b and /3 enable us (even when 

 the laps are equal) to balance, or even more than balance,, 

 the inequality of admission of steam on the two sides of 

 the piston due to shortness of connecting rod. 



It is easy to see how such a diagram may be modified 

 for problems concerning cut off valves on the back of 

 the main slide valve. 



The same expedient of false centres may be used tO' 

 show the velocity or acceleration of a slide. 



In a modified diagram Mr. Harrison sometimes lets 

 the two circles coincide, using two false centres. 



The solution is a close approximation to the truth ir* 

 all the usual cases, because b is always small in com- 

 parison with a. John Perry. 



INSECTS AS CARRIERS OF DISEASE.^ 



THE recent researches of bacteriologists into the ro/e 

 played by insects as carriers of infection, and the 

 hunt after microbes to locate their natural habitat, is a 

 necessary procedure before it becomes possible to enter 

 on a scientific crusade against them. In those diseases 

 which may be caused by infection carried by insects, it 

 is a more hopeful task to deal with the insects which we 

 can see, than to deal with the microbe which lurks un- 

 seen and unheeded. At the same time, it is an uncom- 

 fortable thought that insects which we have regarded as- 

 undesirable but harmless may be the cause of a serious 

 illness. 



The bacteriologist has now shown a fair-sized category 

 of diseases to be caused by microbes, and having arrived 

 so far the hygienist steps in and wants to know firstly,, 

 what is the habitat of these microbes outside the human 

 or animal body if they have one, and secondly, by what 

 means they are conveyed to the body from their resting- 

 place outside or from one patient to another ? 



1 "On the /?(J/f of Insects, Aiachnids and Myriapods, as carriers in the 

 spread of Bacterial and Parasitic Diseases of Man and Animals. A critical 

 and historical study." By Dr. G. H. F. Nuttall (from the Jo/uis Hopkins 

 Hospital Reports, vol. viii.). 



