June 14, 1877] 



NA TURE 



T27 



in a vertical straight line, the distance P C in Fig. 20 being 

 the same as it was in Fig. 19 ; but from a well-known 

 property of a circle, if H be any one of the holes pierced 

 m the piece, the angle H P' P is constant, thus the straight 

 line H P' is fixed in position, and H moves along it ; 

 similarly all the other holes move along in straight lines 

 passing through the fixed pivot P', and we get straight 

 line motion distributed in all directions. This species of 

 motion is called by Prof. Sylvester "tram-motion." It 

 is worth noticing that the motion of the circular disc is 

 the same as it would have been if the dotted circle on it 

 rolled inside the large dotted circle ; we have, in fact, 

 White's parallel motion reproduced by linkwork. Of 

 course, if we only require motion in one direction, we may 

 cut away all the disc except a portion forming a bent 

 arm containing C, P, and the point which moves in the 

 required direction. 



The double kite of Fig. 18 may be employed to form 

 some other useful hnkworks. It is often necessary to 

 have, not a single point, but a whole piece moving 

 so that all points on it move in straight lines. I may 

 instance the slide rests in lathes, traversing tables, 

 punches, drills, drawbridges, &c. The double kite en- 

 ables us to produce linkworks having this property. In 

 the linkwork of Fig. 21, the construction of which will be 



at once appreciated if you understand the double kite, 

 the horizontal link moves to and fro as if shding in a 

 tixed horizontal straight tube. This form would possibly 

 be useful as a girder for a drawbridge. 



In the linkwork of Fig. 22, which is another combina- 

 tion of two double kites, the vertical Unk moves so that 

 all its points move in horizontal straight lines. There is 

 a modification of this hnkwork which will, I think, be found 

 interesting. In the linkage in Fig. 23, which, if the thin 

 links are removed, is a skeleton drawing of Fig. 22, let the 

 dotted links be taken away and the thin ones be in- 

 serted ; we then get a linkage which has the same 

 property as that in Fig. 22, but it is seen in its new form 

 to be the ordinary double parallel ruler with three added 

 links. Fig. 24. is a figure of a double parallel rule made 

 on this plan with a slight modification. If the bottom 

 ruler be held horizontal the top moves vertically up and 

 down the board, having no lateral movement. 



While I am upon this sort of movement I may point 

 out an apparatus exhibited in the Loan Collection by 

 Prof. Tchebicheff which bears a strong hkeness to a com- 

 plicated camp-stool, the seat of which has horizontal mo- 

 tion . The motion is not strictly rectilinear ; the apparatus 

 being, as will be seen by observing that the thin line in 

 the figure is of invariable length, and a link might there- 

 fore be put where it is, a combination of two of the parallel 



motions of Prof. Tchebicheff given in Fig. 4, with some 

 Imks added to keep the seat parallel with the base. The 

 variation of the upper plane, from a strictly horizontal 

 movement is therefore double that of the tracer m the 

 simple parallel motion. 



Fig. 26 shows how a similar apparatus of much simpler 

 construction employing the Tchebicheff approximate 

 parallel motion can be made. The lengths of the links 

 forming the parallel motion have been given before 

 (Fig. 4). The distance between the pivots on the moving 

 seat is half that between the fixed pivots, and the length 

 of the remaining link is one-half that of the radial links. 



An exact motion of the same description is shown in 

 Fig. 27. O, C, O', P are the four foci of the quadriplane 

 shown in the figure m which the links are bent through a 

 right angle, so that O C ' O P is constant, and C O P a right 

 angle. The focus O is pivoted to a fixed point, and C is 

 made by means of the extra link Q C to move in a circle of 

 which the radius Q C is equal to the pivot distance O Q. 

 P consequently moves in a straight line parallel to O Q, 

 the five moving pieces thus far described constituting the 

 Sylvester-Kempe parallel motion. To this are added the 

 moving seat and the remaining link R O', the pivot dis- 

 tances of which, P R and R O', are equal to O Q. The 

 seat in consequence always remains parallel to O O, and 

 as P moves accurately in a horizontal straight line, every 

 point on it will do so also. This apparatus might be used 

 with advantage where a very smoothly-working traversing 

 table is required. 



{To be continued.) 



SPONTANEOUS GENERATION' 

 'T'HE investigation embodied in the memoir now 

 -*• .submitted to the Society was opened in the 

 summer of 1876 by a series of tentative experiments 

 en turnip-infusions, to which were added varying quan- 

 tities of bruised or pounded cheese. I was soon, how- 

 ever, drawn away from them to other experiments on 

 infusions of hay. With this substance no difficulty was 

 encountered in my first inquiry. Boiled for five minutes, 

 and exposed to air purified spontaneously or freed from 

 its floating matter by calcination or filtration, hay in- 

 fusion, though employed in multiplied experiments at 

 various times, never showed the least competence to 

 kindle into life. After months of transparency, I have, 

 in a great number of cases, inoculated this infusion with 

 the smallest specks of animal and vegetable liquids con- 

 taining Bacteria, and observed twenty-four hours after- 

 wards, its colour lightened, and its mass rendered opaque 

 by the multiplication of these organisms. 



But in the autumn of 1876, the substance with which I 

 had experimented so easily and successfully a year 

 previously, appeared to have changed its nature. The 

 infusions extracted from it bore in some cases not only 

 five minutes' but fifteen minutes' boiling with impunity. 

 But on changing the hay a different result was often 

 obtained. Many of the infusions extracted from samples 

 of hay purchased in the autumn of 1S76, behaved exactly 

 like those extracted from the hay of 1S75, being com- 

 pletely sterihzed by five minutes' boiling. 



To solve these discrepancies, numerous and laborious 

 experiments were executed with hay derived from different 

 localities, and by this means in the earlier days of the 

 inquiry, it was revealed that the infusions which mani- 

 fested this previously unobserved resistance to steriUza- 

 tion were, one and all, extracted from old hay, while the 

 readily sterilized infusions were extracted from new hay, 

 the germs adhering to which had not been subjected to 

 long-continued desiccation. 



I then fell back upon infusions whose deportment had 



^ " Further Researches on the Deportment and Vital Resistance of Putre- 

 f-ictive and Infective Organisms, from a Physical Point of View." By John 

 lyndall, LL.D., F.R.S., Professor of Natural Philosophy in the; Royal 

 1 natitutioa. —Abstract. 



