February, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



33 



slowlv, hut completely, hv hot water, bv cold dilute 

 hydrochloric acid, or by heating it to iio°C. 



The recent action of two or three large 

 The Metric manufacturing firms has brought this 

 System of subject prominently to the front. 

 Weights and Photographers are peculiarly unfortu- 

 Measures. nate in having to use an ounce that is 

 divided into 437 1 grains, a figure that 

 no one can suggest is convenient for division, much 

 less sub-division. The figure is impossible, and other 

 figures, such as 440, 450, or 480, are substituted for it 

 when necessary, in spite of the error involved. The 

 metric system appears to be the one practical way out 

 of this and many other difficulties. But the present 

 metric system is founded upon arbitary stardards, and 

 so is fundamentally no better than the English systems. 

 It appears that when the metre was defined, those in- 

 terested set to work to make a model of it, and when 

 later the model was found to be a little wrong, the real 

 standard was neglected, and the incorrect model 

 adopted instead of it. .A vessel to hold a litre was 

 also made, and this was sub.sequently found not to hold 

 a cubic decimetre as it should, but instead of correcting 

 the error, that particular vessel was made the standard. 

 1 suggest that we want only a standard of length, and . 

 thatlhat should be defined, if possible, without reference 

 to any particular bar of metal. There must, of course, 

 Ix; secondary standards of length, weight, and capacity, 

 and from these other standards are needed to make the 

 measures from that are used in every-day work. But 

 I would suggest that all secondary and other standards 

 should l>e regarded merely as practical conveniences, 

 and if found to vary from the standard, should either be 

 corrected or so marked that their error may be allowed 

 for. Such errors as these would only exceptionally be 

 of importance, and would, of course, be known to all 

 concerned. The disadvantages of having standards of 

 kngth, weight, and capacity not simply related to each 

 other are so obvious that they hardly need pointing out. 

 We have, for example, already in the metric system two 

 standards of capacity, the litre and the cubic decimetre, 

 and we shall perhajjs get half-a-dozen others as time 

 goes on, unless we mend our ways. The trouble of 

 these many competing units is shown in the disregard 

 of their differences even in scientific work. 



That portion of dust that cannot be 

 Laying the kept out of scientific instruments, such 

 Dust in as cameras, it is well to trap or catch. 

 Cameras. that its oresence may do as little harm 

 as possible. One method recommended 

 for this purpose is to smear the wood-work with a 

 trace of glycerine, so that whatever dust comes into 

 contact with it, sticks to it. Doubtless this is effective, 

 but the method does not commend itself to many 

 because of the trouble of cleaning the dust-laden sur- 

 face. Velvet catches dust and holds it lenatiously, it 

 is not messy, it does not need renewal, like glycerine, 

 and it is not very difficult to clean by brushing. If 

 black velvet is used, its light-absorbing power is an 

 additional advantage as a lining for optical instruments. 

 In all cases where dustless air is required in order to 

 prc\ent as far as possible the scattering of light, black 

 veh ;'t offers the double advantage of being the best 

 non-rjflccting surface as well as an eflicient dust-trap. 



" The Photo-Miniature," to hand, almost 

 Received. a year after the date it bears, deals 

 with the " Hand Camera," and gives a 

 considerable amount of practical information. 



MaLximvirrv of Mira^ Ceti. 



By P. M. Ryves. 



The variable star Mira (o Ceti) has lately passed an 

 unusually bright ma-\imum, having remained for three 

 weeks fully second magnitude, which is about the same 

 as at the time of its discovery by David Fabricus in 1595. 

 When observed on July 30 last, the brightness was 

 estimated as just under the ninth magnitude. It in- 

 creased, at first slowly then more rapidly, and reached 

 the seventh magnitude by October 17. About this 

 date a more rapid rise set in ; the star rushed up to the 

 second magnitude by December 2, a change corre- 

 sponding to an increase in light of a hundred times in 

 less than fifty days. The most rapid portion of this rise 

 was from the 6th to the 3rd mag. in 19 days (October 

 26-November 14). 



A similar rapid rise to maximum occurred last winter 

 and is, in fact, a characteristic feature of the variation of 

 this and certain other of the variable stars of long period ; 

 but it is not always so well marked as in the present 



Light curve of Atira (o Ceti) 1906-7 compared with previous maximum. 

 Each curve is based upon about 40 observations. 



case. This star may only reach the 5th mag. at maxi- 

 mum, and frequently does not exceed the 4th mag., but 

 when a bright maximum, like that of December last, 

 occurs, Mira surpasses in brightness any other variable 

 star of regular period. 



On account of the general interest which attaches to 

 this variable and also the ease with which it may be 

 observed, it usually receives a good deal of attention 

 from amateurs when the epoch of maximum falls at a 

 season of the ye.tr when it can be conveniently observed. 

 Unfortunately, the position of Mira south of the ecliptic 

 and the period which brings the maxima about a month 

 earlier each year render it impossible to observe the 

 maxima well for several years together when they fall 

 in the spring arid summer months. A good series of 

 observations, extending well on either side of the epoch, 

 can only be obtained in the case of about half the 

 maxima, and those observers who do not work after mid- 

 night will he restricted to a still smaller proportion It 

 is, therefore, very desirable that a careful watch should 

 be kept upon this variable during the present and next 

 few years while the conditions are favourable. .\ special 



