18+8.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



39 



Let the circle (diagram 6) represent the crank's orbit, 

 diameter ab the stroke of the piston, to some known scale. 



and its 

 Make 



Diagram 6. 



oc equal to the part of the stroke performed before the steam is 

 out off; and bd equal to the distance of the piston from the end of 

 its stroke when steam is re-admittpd for the return stroke. Draw 

 d e and cf at right angles to a fc, and mark the point g at the dis- 

 tance /> e from ./". Bisect the arc o 7, and from the point of bisec- 

 tion, A, draw the diameter li i, JMake I k ei|ual to be ; draw i m and 

 k I at right angles to a fc ; and draw ;'/ and i 4 indefinitely. From 

 the point m, set off m n equal to the width of steam port, full size ; 

 from n draw n parallel to i m, and meeting i h , and also op pa- 

 rallel to a 6, and meeting /i ? : then will sp equal the lap, and sr 

 the lead. 



In all the foregoing cases, we have taken the versed sine of the 

 arc described by the crank, from either extremity of the stroke, as 

 the portion of the stroke performed by the piston ; but, as has 

 been already observed, the relative positions of the piston and 

 crank depend upon the length of the connecting-rod, which will 

 be seen by reference to diagram 7, where A B represents the stroke 

 of the piston, C D the connecting-rod, and D O the crank Now, 



Diagram 7. 



by supposing a rf to be the arc described by the crank when the 

 piston has performed one-fourth of its stroke, and from the length 

 of that arc, calculating tlie amount of lap required to cut off the 

 steam at that part of the stroke, we appear to be in error — for, 

 from the oblique action of the connecting-rod, the piston would 

 have descended only to the point c. But the engine being double- 

 acting, we have to take into consideration the position of the crank 

 when the piston has performed one-fourth of its stroke in the 

 opposite direction from the point B : and here we find, that by 

 supposing the crank to have described the arc be (equal to arf), 

 instead of the true arc b E, we cause the steam to be cut off when 

 the piston has reached the point f; and the distance By' being 

 precisely as much more than B F as A c is less than AC, tlie seem- 

 ing error is self-corrective. 



A Table of Multipliers to find the Lap and Lead, when the Steam in 



to be cut off at ^ to \ths of the Stroke. 



The Lap must lie equal to the width of the steam port multiplied by Col. 1. 



The Lead must be equal to the width of the steam port multiplied by Col. 2. 



Example of its application. — Stroke 36 inches ; width of port 2 



inches ; steam to be cut off at half-stroke ; distance of the piston 



from the end of its stroke when steam is re-admitted for the 



return stroke, 1-5 inches. 



1^5 



— = 0833. Find that number, or the one nearest to it, in 

 18 



the right-hand or last column, and take out the multipliers on tlie 



same line under the head Half-stroke. 



Then 2 x 1-21 = 2^+2 inches = the lap. 



And 2 X -638 = 1-276 inches = the lead. 



R. B. C. 



HEALTH OF TOWNS COMMISSION. 



We may seem to be rather late in noticing the first report of 

 the Metropolitan Sanitary Commissioners, but the first nimiber of 

 our new volume was so filled with other matter, that we were unable 

 to do more than to call attention to the unfair way in which the 

 profession has been treated by the commissioners and the govern- 

 ment. Since then we are sorry to find that the design of employ- 

 ing military engineers in making the survey of London is persisted 

 in, and that at a time when numbers of experienced and well 

 qualified surveyors in the metropolis are without employment. 



The first part of the Report is devoted to a consideration of the 

 means necessary to resist the cholera. After a careful investiga- 

 tion, they come to the conclusion, which appears to us to be well 

 founded, that cholera is not contagious, and that the great means 

 of lessening its ravages are to be found in improved sanitary 

 arrangements, particularly in connexion with the sewage. 



To improve the sewage is their first step, and they have recom- 

 mended and obtained the revocation of the old commissions of 

 sewers. This is a measure to which we have already given our 

 strongest advocacy, but we do not think that the commissioners 

 have gone far enough. The Regent-street and Regent's-park dis- 

 trict remains a narrow slip, running up from the Thames across 

 the drainage of the Westminster and Holborn districts, and having 

 a grand and deep sewer of nearly the capacity of the Fleet, whicn 

 being employed as an outfall, would as we have before pointed out 

 be immediately available in improving the drainage of a very large 

 district. It is true that this is under the virtual jurisdiction of 

 the Commissioners ot Woods and Forests, but the commission 

 ought to be at once revoked, and the jurisdiction transferred to 

 the new metropolitan commissioners. The maintenance of this 

 commission by the government is a reason which will be used for 

 the maintenance of the City of London Commission, which is like- 

 wise left untouched, because, as the commissioners say, they have 

 not had time to look into the case, but because, as we presume, Mr. 

 Lambert Jones prevented it, and because the commissioners did 



