PEN 



PEN 



lower end of the bar rests upon a fixed 

 point, the bar will be expanded upwards, 

 and raise the upper end of the pendulum 

 just as much as its length was increased ; 

 and hence its length below the plates 

 will be the same as before. Of this pen- 

 dulum, somewhat improved by Mr. 

 Crosthwaite, watch and clockmaker, Dub- 

 lin, we have the following description, 

 " A and B (fig. 12), are two rods of steel 

 forged out of the same bar, at the same 

 time, of the same temper, and in every 

 respect similar. On the top of B is form- 

 ed a gibbet C ; this rod is firmly support- 

 ed by a steel bracket D, fixed on a large 

 piece of marble E, firmly set into the wall 

 F, and having liberty to move freely up- 

 wards between cross staples of brass, 1,2, 

 3, 4, which touch only in a point in front 

 and rear (the staples having been care- 

 fully formed for that purpose) ; to the 

 other rod is firmly fixed by its centre the 

 lens G, of twenty-four pounds weight, 

 although it should in strictness be a little 

 below it. This pendulum is suspended 

 by a short steel spring on the gibbet at 

 C : all which is entirely independent of 

 the clock. To the back of the clock- 

 plate, I, are firmly screwed two cheeks 

 nearly cycloidal at K, exactly in a line 

 with the centre of the verge L. The 

 maintaining power is applied by a cylin- 

 drical steel-stud, in the usual way of re- 

 gulators at M. Now, it is very evident, 

 that any expansion or contraction that 

 takes place in either of these exactly 

 similar rods, is instantly counteracted by 

 the other; whereas in all compensation 

 pendulums composed of different mate- 

 rials, however just the calculation may 

 seem to be, that can never be the case, 

 as not only different metals, but also dif- 

 ferent bars of the same metal, that are 

 not manufactured at the same time, and 

 exactly in the same manner, are found by 

 a good pyrometer to differ materially in 

 their degrees of expansion and contrac- 

 tion, a very small change affecting one 

 and not the other." The expansion or 

 contraction of straight-grained fir-wood 

 length wise, by change of temperature, is 

 so small, that it is found to make very 

 good pendulum rods. The wood called 

 sapadillo is said to be still better. There 

 is good reason to believe, that the pre- 

 vious baking, varnishing, gilding, or 

 soaking of these woods in any melted 

 matter, only tends to impair the property 

 that renders them valuable. They should 

 be simply rubbed on the outside with 

 vvux and a cloth. In pendulums of this 



construction the error is greatly diminish- 

 ed, but not taken away. 



PENGUIN. See AI-TENODYTES. 



PENELOPE, in natural history, a ge- 

 nus of birds of the order Gallina. By La- 

 tham, they are mostly arranged under the 

 genus Meleagris, or the Turkey. Their 

 legs, however, are without spurs. They 

 inhabit principally South America, and 

 particularly Brasil and Guiana. The P. 

 cristata, or guan, is two feet six inches in 

 length. P. cumanensis, or the yacou, 

 is of the size of a hen turkey, and is 

 found in Cayenne and Guiana. The Ma- 

 rail is found in flocks in Guiana, feeds on 

 fruits, and roosts on trees. See Aves, 

 Plate XI. fig. 5. 



PENIS. See AXATOMY. 



PENNANT1A, in botany, so named hi 

 honour of Thomas Pennant, a genus of 

 the Polygamia Dioecia class and order. 

 Essential character : calyx, none ; corolla 

 five petalled ; stamens five : pericarpium, 

 three sided, two-celled, with solitary sub- 

 triquetrous seeds. There is but one spe- 

 cies, viz. P. corymbosa, a native of New 

 Zealand. 



PENNATULA, in natural history, sea- 

 pen, a genus of the Vermes Zoophyta 

 class and order ; animal not affixed, of va- 

 rious shapes, supported by a bony part 

 within, naked at the base, the upper part 

 with generally lateral ramifications, fur- 

 nished with rows of tubular denticles 

 producing radiate polypes from each 

 tube. There are about eighteen species, 

 of which P. coccinea is described as 

 stem round, radiating, with papillous po- 

 lype-bearing sides, and clavate at the top. 

 It is found in the White Sea, is soft, red, 

 an inch and a half high, and as thick as 

 the litcle finger, wrinkled, with the pa- 

 pillae disposed in rows. P. phosphoreu. 

 has a fleshy stem, with a rough midrib, 

 and imbricate ramification. It inhabits 

 most seas, and emits a very strong phos- 

 phoric light in the dark ; about four 

 inches long, red, stem villous, with a lan- 

 ceolate rough midrib, and nearly incum- 

 bent rays, the tubes pointing all one way 

 P. reniformis : stem round, vermicular, 

 supporting a kidney-shaped leaf-like head, 

 producing polypes on one surface. It in- 

 habits South Carolina : body expanded, 

 kidney-shaped, flat, rising from a shore 

 round stem, and covered on the upper 

 surface with numerous tubular orifices, 

 through which the polypes are obtruded 

 at pleasure ; the upper surface is of a rich 

 purple, the underside brilliant, and some- 

 times yellowish. 



