172 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[May, 



we find tliat there was another and more serious danger to guard 

 against, and that without the assistance of the coffre dam, the sunken 

 pier could /itctr have been made secure— on tlie contrary, that any at- 

 tempt by driving piles or otherwise, if access could not have been had 

 to the interior might liave proved fatal to the adjoining arches. 



We hope soon to have it in our power to announce, that the com- 

 niissioners sec that the time has arrived when they may confer a great 

 and lasting benefit on the public, by widening the roadway of the 

 bridge, "a consummation devoutly to be wished," by every one 

 who has occassion to pass over it in its present narrow and crowded 

 state. 



ON THE POWER OF THE SCREW. 



Sir — Permit me to offer you the following article which i hope you 

 vill deem worthy of publication in your Journal. 



I am your most obedient servant, 



Bfuff, ' i- R- CuisEX. 



March 30, 1840. 



I liave been often consulted as to the application of the screw as a 

 mechanical power, and frequently found theory at variance with prac- 

 tice, this le<l me to an investigation of the rule generally used, for 

 calculating its power, to practical trials of its power, and to an eluci- 

 dation of a rule diflerent from all those I know to be in use, which I 

 trust will be found correct. 



The Rev. Mr. Bridges in his work on Mechanics, p. 25.1, states, that 

 P : W : : rf : cirf. of cylinder, d being in his words the distance 

 between two threads of the spiral, in p. •l'^', he says that^ : W : : 

 d : cirf.of cyUnder,andP ; p : : cirf.cylinder : cirf. of circle f.r<f5«o 



2;jaP 

 P : W : : d : cirf. of circle • "' — 



.w 



d 



a z= length of lever, but 



the 



tlie nut, the tlireads of the nut pass over the same space ou the threads 

 of the screw, and both (/. c. the threads of the nut and the threads of 

 the screw) sustain equal parts of the weight or pressure. 



The power gained by the Rev. Mr. Bridge's formula by taking cre- 

 dit for the circumference of the lever, and dividing by but half the 

 elevation of the inclined plane, is more than lost by omitting the ad- 

 vantage gained by the inclined plane in laige screws, and the power 

 of small diameter screws is overrated. 



I am convinced that the true basis for calculating the power of the 

 screw is P ; W :: d : circumference of cylinder, (i being the height 

 of the inclined plane or the elevation or depression obtained by each 

 revolution of the cylinder, then this advantage multiplied by the power 

 applied, and the product divided by the height of the inclined plane, 

 that is. 



As the elevation obtained at each revolution, or as the height of the 



inclined plane, 

 : circumference of the cylinder, 



; ; the power applied 

 ; the weight or pressure, 



P X circumference of cylinder , - , 

 or W =; T- '— ; the formula most generally 



wanting in use. 



.Suppose three screws, each of i inch thread, worked by a lever 90 

 inches long, the lever moved by a windlass of one ton power, the screws 

 to be of 3, '', and 9 inches diameter, we have the weight raised or 

 pressure produced, by the 



lie makes ;; = 3-1415, and also ;j= the power acting on the surface of 

 the cylinder, thus making p in the same equation variable and invaria- 

 ble, 'in his application of the above formula he uses/y = 3-141.i, but 

 emits ;^= power acting on the surface of the cylinder, he adds in a 

 note (6) that P : W : : rf : cirf. of the circle. Whatever he the 

 thirhita-i of the cylinder on which the seven' is cut. He then gives this 

 rule. The power necessary to sustain the weight or produce the 

 pressure will ahvavs bear to that weight or pressure, the ratio of the 

 distance between any two spirals of the screw to the circumference 



(2;; <iP\ 

 W = — - — I 



■weight to be raised or pressure produced is equal to twice the radius 

 of the lever X 3-1415 X the power applied, and this product divided 

 bv the distance between the threads. 

 ' The first objection that struck me was why d should represent the 

 distance between the spirals, and not the elevation of the inclined 

 jilane, or the elevation or depression obtained by each revolution of 

 the cylinder, this is generally the distance between two threads + the 

 thickness of the thread, or twice the distance between two threads; 

 it is obvious that if the thread be i inch, and the distance between the 

 threads i inch, that the elevation of the inclined plane, or the eleva- 

 tion or depression obtained at each revolution of the cylinder will be 

 one inch. 



The second objection was to the deduced conclusion that the diame- 

 ter of the cvlii.der was of no importance, or that a screw of 2 inches 

 diameter was as powerful as one of 12 inches or 100 inches. Suppose 

 that no lever is used, and that the thread is tlie same in eicb, say k 

 inch, and that the advantage obtained by the inclined plane be calcu- 

 lated we have for the 2 inch diameter screw 1 : (>2S3 : I P : M', 

 and fur the 12 inch diameter 1 : 37'G;iS : : P : W, that is, the 12 

 inch diameter considering it merely as an inclined plane will sustain 

 in equilibrio sis times the weight with the same power that the 2 inch 

 diameter screw will sustain. 



That this power or advantage could be lost by the application of 

 the same lever is absurd. 



The third objection was to multipl\ ing by the circumference of the 

 circle formed by the extremity of the lever, instead of by the radius of 

 the lever, as well may the circumference described by every lever be 

 calculated, the error in calculating the power of the wheel and axle by 

 the circumference would be apparent; in fact a screw is but a re- 

 volving inclined plane. Motion and power being communicated to it 

 by a lever ; moreover this inclined plane is properly speaking a fox- 

 wedge, that is, two inclined planes of equal height acting on each 

 Other, for whatever space the threads of the screw pass over that of 



tons. 

 90X1 : 848-205 

 ;>Oxl : 1696-41 

 90X1 : 2546-15 



3 inch diameter screw thus 1 : 9-4-245 

 by the l5 ditto 1 : 18-849 



by the 9 ditto 1 : 28-2735 



By the Rev. Mr. Bridge's formula we liave for the three but one 



2^aP . „, 2x3-1415x 90X1 ,,„r,„,. 

 "^ ' ' >^' — . — - — = 1130-94 tons. 



2 



power, W 



, i. e. W 



It is to be remembered that one-tliirdof the calculated power of the 

 screw is lost by friction. 



It is my opinion that the screw could be made to supersede the 

 capstan in patent slips and dockyards, and that it could be used to the 

 greatest advantage in submarine operations and excavations: its prac- 

 tical application to these objects will form the subject of another 

 article. 



COMPETITION. 



Sir — A very suspicious looking advertisement haviug appeared iu the 

 Times of the 12th instant, offering a premium of 20/. lor deigns, estimates 

 and specifications for a clnnch to hold SOO or 1000 persons, to be built at 

 Turnham Green, 1 applied according to the directions giveu iu tLe advertise, 

 meiit, for informatiou upon two or three points of some importance, viz. how 

 nmcli money it is proposed to exjiend — what means the advertisers would 

 take to ascertain th.it the accejiteil design could be executed for the estimate 

 which accompanied it — and whether the successful candidate would be em- 

 ployed in case he proved to be an architect of good reputation and experi- 

 ence. In answer to which queries I am informed, 

 " That the site is level and the soil gravel — 



Tliat the expenditure is not to exceed J£3,500— ' 



That one-third of the sittings are to be free — 



That no vaults are required — 



And tliat these are the only additional particulars the Secretary to tiie 

 Conmnttec can furnish." 



Perhaps you can tied room to publish this informatiou for the benefit of 

 the profession. 



I am, Sir, vour obedient senaat, 



H. T. 



Jpril 19, 1841. 



Inclosed is my name and address. 



Sh-iim Navigation Iv tlif Weil /flrfiVs.— Tlie first of the splendid line of steam 

 packets intended to carry the mails betwixt this country and ihe West Indies, 

 has been launcluil from tho building-yacd of Messrs. Duncan and Company, 

 at Greenock. This vessel, which is IfjOO tons hurlhen, has been named TIte. 

 Cli/fit: and is described as having a most perfect model. Her ensines, made 

 by Messrs. Caird and Co., are ni readiness, and will he put in witliout delny. 

 lliere are at present six of this line of packets, all of die same tonnage, build- 

 ing on the Cly<!e ; four at Greenock, one at Port-Glasgow, and one at Dum- 

 barton ; and there is also one at Leilh. — Glasgoiv Argus. 



