no 



BANG'S PLATOMETER, OR SELF A.CTING CALCULATOB OF SURFACE. 



I I-.. i 



indications of this increase of destiny; Bomeofthe Actinia will 



remain closed and bi me coated with a white sjimy covering 



within' which they remain for a length of time, and if 

 specific gravity of the water be lowered this is very Boon ruptured 

 by their expansion, thrown off, and the tentacula become soon 

 extended. 



All putrescent matter or exo is of food or rcjecta of the Actinia 

 should l>u carefully removed from the water, as the noxious 

 gaseous compounds generated bj the decay ofsuch matters appear 



tp dill'iis' themselves rapidly through tin- water, art a- ;i virulent 



poison, and speedily destroy t Ii* - vitality of the occupants. Thus 

 many beautiful subjects were lost in a few hours from the intro- 

 duction, into a small glass jar of a large Pecten shell, encrusted 

 with corallines, which had become I ■ ■. i< I ■ -■ 1 with putrescent matter 

 by a partial submersion in a foul muddy bottom. 



Great care should also be taken in moving the Actinice, that 

 the foot or sucking disc with which it attaches itself to the rocks, 

 stones, or mud, be not injured, as, when this occurs, they rarely 

 survive, but roll about without attaching themselves, and gradu- 

 ally waste away and die. 



"With these exceptions then, everything lias gone on very 

 satisfactorily, care being always taken not to overload the water 

 with too large a proportion of animal life for the vegetation to 

 balance, as whenever this has been inadvertently attempted, the 

 water has soon become foul, and the whole contents of the 

 tank, both animal and vegetable, have rapidly suffered, and it 

 has required some time before the water could be restored to its 

 former healthy condition. 



In one of the numbers of the " Zoologist" of last year, I stated 

 that besides the Ulvce, Enleromorphce, and Cladop/iorce, I had 

 found the Zostera marina a very useful plant for oxygenating 

 the sea water; but this observation has reference only to the 

 case of a tank supplied with a ground where its roots will find 

 a sufficiency of food for its growth, as in a clear shingle or sand 

 it soon decays; and it should be associated with such animals as 

 delight in a ground of this nature, as many of the Annelids, 

 Crabs, burrowing Shrimps, etc. There are several interesting 

 observations which have been made from time to time connected 

 with this subject, which I hope to lay before the natural-history 

 world as soon as I can find leisure time for the purpose. 



Sang's Platometer, or Self-acting Calculator of Surface. 



In this number we publish a description (see page 305), ac- 

 companied with plate, of an exceedingly ingenious instrument 



"^^^sts* /ir*K\ 



for computing the areas of irregular figures. One of these in- 

 struments has recently been imported from England for the ser- 



vice of an engineering establishment in town, and we have been 



i' ed with an o of examining its mechanism a 



g its accui 



By the usual and well known method of dividing the figure 

 to bi ired into a number of triangles or trapeziums, i are 



cert ai nil the base and altitude of each, and taking the 



Minis of the product-, the area may be (1 v.itli great aC- 



curac - it is necessary to revise the calculations several 



limes, both for the purpose of obviating fault in the arithmetical 

 part of the work, and in order, by taking the average of a few 

 independent measurements, to increase the probable accuracy of 



the result, this method of calculation, especially when the figure 

 is irregular, entails a considerable amount of labour of an irksome 

 kind. Attempts have been made to avoid this by cutting the 

 figure from the sheet of paper, and weighing it in a delicate ba- 

 lance against weights, consisting of part of the same paper of de- 

 terminate sizes; but this method — at firstsight simple and prac- 

 tical — is rendered of little use by the impossibility of obtaining 

 paper of uniform thickness throughout the sheets, the variations 

 of thickness, and hence of weight, being greater than the amount 

 of error that could be allowed in the results. 



The platometer indicates the area of any figure, however irre- 

 gular, on merely carrying the point of a tracer round its bound- 

 ary; and, besides the advantage of not injuring the drawing, it 

 possesses that of speed and accuracy. 



From the peculiar construction of the instrument, it Ls appa- 

 rent that if the tracer be moved forward, it will cause the index 

 to revolve, not simply in proportion to that motion, but in pro- 

 portion to the motion of the tracer multiplied by the distance of 

 the edge of the index wheel from the apex of the cone ; and that 

 the revolving motion of the index wheel will be positive or ne- 

 gative, according as the tracer is carried backwards or forwards. 

 Hence, if the tracer be carried completely round the outline of 

 any figure — on arriving at the end of its journev, the index wheel 

 - will show the algebraic sum of the breadth of the figure at every 

 point, multiplied by the increment of the distance of the points 

 from the apex of the cone ; that is to say, the area of the figure. 



As pointed out in the general description of the instrument, 

 the exact amount of any errors which may arise from imperfec- 

 tions in mechanism or adjustment can easily be discovered, by 

 simply reversing the paper and moving the tracer a second time 

 over the boundary of the figure; if the results of both trials be 

 alike there can be no error; but if they vary [the errors in one 

 case being positive, and in the other negative], an average be- 

 tween the two is the exact area of the figure, and is more to be 

 depended on than the results of measurements made by scale and 

 calculation in the usual way. A careful operator, iu using the 

 platometer, will always take the average of two tracings as de- 

 scribed; but when he experiences the rapidity with which this 

 may be done, he will find the trouble as nothing in comparison 

 with the harassing labour of calculating by scale and multiplica- 

 tion. 



Sang's Platometer, like most other really valuable inventions, 

 possesses great simplicity of construction, and is not liable to get 

 out of order with ordinary care. 



It can be ordered from the inventor through Messrs. Hearn 

 <fc Potter, mathematical instrument makers, Toronto. 



