October 2, 1885.] 



SCIENCE 



283 



LETTERS TO THE EDITOR. 



%* Correspondents are requested to he as brief as possible. The 

 tcriter's name is in all cases required as proof of good faith. 



Electric measuring apparatus. 



Iis^ the American journal of scienceior March, Pro- 

 fessor Trowbridge describes a form of differential 

 cosine galvanometer, in which the action on a magnet 

 of a strong current, moving through the fixed vertical 

 circle of a large tangent galvanometer (of one metre 

 radins), is balanced by the opposite effect of a weak 

 current from a Daniell cell moving in a reverse direc- 

 tion through the coil of a cosine galvanometer, the 

 fixed and movable coils having a common centre. 

 By moving the coil of the cosine galvanometer about 

 its horizontal axis, it is easy to secure a balance, 

 and thus to determine the magnitude of the strong 

 current. An obvious and simple modification of this 

 apparatus consists in substituting for the cosine gal- 

 vanometer an ordinary tangent galvanometer, with a 

 coil of small radius having a number of turns of 

 wire in circuit with a battery and rheostat. By 

 varying the resistance in the circuit, a balance can 

 be reached, and the strength of the current found. 

 A mirror galvanometer thus arranged, and in direct 

 circuit with a battery and very high resistance, or in 

 derived circuit with a battery and tangent galvanom- 

 eter, might sometimes be useful, as in studying slow 

 variations in strong currents. Another instrument, 

 which is likely to prove valuable for measuring strong 

 currents, is a new form of differential cosine galva- 

 nometer, recently devised by Mr. E, H. Pierce, while 

 a student at the Massachusetts institute of technol- 

 ogy. The current is caused to pass in opposite direc- 

 tions through two concentric circles of nearly the 

 same radius, as in Brackett's differential galvanom- 

 eter; but the inner of these is capable of moving 

 upon a horizontal axis, as in the ordinary cosine gal- 

 vanomoter, and it is revolved until a convenient de- 

 flection is secured. A simple formula then gives the 

 strength of the current. Chas. K. Ckoss. 



The magnetic declination in 1728. 



I notice, in your issue of Sept. 18, a note, over the 

 signature ' C. A. S.,' criticising certain statements 

 in regard to the northern boundary of North Caro- 

 lina, contained in bulletin No. 13 of the U. S. geolo- 

 gical survey. A more careful perusal would have 

 shown the writer that the points criticised are con- 

 tained in an extract from ' Geology of North Caro- 

 lina,' by Prof. W. C. Kerr, 1875, and that the author 

 of the bulletin is not responsible for them. 



Henry Gannett. 



Washington, D.C., 

 Sept. 21. 



Composite portraiture. 



Shortly after the publication of my article on 

 'Composite portraiture' {Science, Aug. 28), my at- 

 tention was called to an article by Mr. W. E. Deken- 

 ham in the Photographic news of April 24, 1885, 

 wherein is figured an arrangement for combining 

 optically more than two photographs, which corre- 

 sponds very nearly with what I had in mind when I 

 penned the last sentence of my article. I have since 

 thought of other methods of solving the problem, 

 but have not had the opportunity of putting them to 

 a practical test. I was glad to find in the same jour- 

 nal (April 17) that Mr. Galton had been giving his 

 attention to the same subject. He says, " There is 

 nothing respecting composites that I should more 



gladly hail than the invention of a simple optical 

 method of combining many images into one, so as to 

 judge of the effect of a photographic composite be- 

 fore making it." 



I also omitted to refer in my article to an im- 

 portant application of the stereoscopic metliod of 

 combining two images; namely, in getting the co- 

 composite of two other composites, as Mr. Galton 

 does in his ' Inquiries into human faculty ' (frontis- 

 piece), and more recently, in his composites repre- 

 senting the Jewish type, Joseph Jastrow. 



Philadelphia, Sept. 18. 



Lower Silurian fossils at Canaan, N.Y. 



The words ' these limestones,' in the report of Pro- 

 fessor Hall's remarks on my paper at tlie Ann Arbor 

 meeting of the American association, published on 

 p. 220 of Science for Sept. 11, seem to imply that Pro- 

 fessor Hall claimed to have known forty years since 

 of Lower Silurian fossils in the Taconic limestone of 

 Canaan, Columbia county, N.Y. In a recent letter 

 to me, he states, that, in the expression, he had no 

 intention of implying that he knew at that time of 

 fossils at Canaan; that he referred to his knowledge 

 of fossils at Hoosic, in a limestone which he regarded 

 as of the same age with that of Canaan. The exist- 

 ence of fossils at Hoosic also, forty miles north of 

 Canaan, is additional evidence with regard to the 

 Lower Silurian age of the rocks of the original Ta- 

 conic system of Emmons. J. D. Dana. 



New Haven, Sept. 22. 



Aquatic respiration of soft-shelled turtles. 



In your otherwise excellent abstract of our paper 

 on the aquatic respiration of soft-shelled turtles 

 {Science, vol. vi. p. 225), not quite enough information 

 is given in the paragraph containing the table to 

 sufficiently explain it. We should be glad to have 

 that paragraph read: " The following table shows 

 the results of the analyses. In the first column is 

 given the total amount of free oxygen taken from the 

 water (10 litres) in ten hours by a turtle weighing 

 1 kilogram. The second column contains the quan- 

 tity of carbon dioxide that could be formed from this 

 oxygen; and the third column contains the actual 

 amount of carbon dioxide added to the water by the 

 turtle, the excess of which, over the amount that 

 could be formed from the oxygen taken from the 

 water, is given in the fourth column." 



July 11 

 Aug. 8 

 Aug. 9 



71 mg. 

 32 " 



CO2. 



97f mg. 

 44 " 

 53f «' 



Actual CO,.|Excess CO, 



231 mg. j ISSf mg. 

 212.7 " 168.7 " 



168.7 " llbfo " 



S. H. AND S. P. Gage. 



AMERICAN FLASH LANGUAGE AGAIN. 



Since writing the paper on this subject 

 which appeared in Science^ vol. v., p. 380,1 

 have succeeded in finding another glossary 

 similar to that given b}' Henry Tufts, about 

 1798, and included in that paper. I am in- 



