I02 A?neiica?i Quarterly Microscopical Journal. 



(2) That the periodic errors of a screw, especiall}^ t 

 which are a function of an entire revolution, vary with the 

 temperature. 



(3) That every screw which acts against springs, in which 

 oil is used as a lubricant, has a variable pitch. 



The first named of these conclusions has been variously criti- 

 cised, notably, and I may add, somewhat ungraciously, by Mr. 

 Webb of London, but it is true, nevertheless. I have no taste for 

 the style of discussion in which Mr. Webb indulged, otherwise, 

 I should have made some reply to his criticisms. Instead, I 

 began anew the experiments on which the conclusion was 

 founded, thinking I might, after all, have been mistaken. This 

 is not the place for a discussion of this point, but I will give 

 one illustration. In seeking for the errors of the screw of my 

 comparator for short lengths, Mr. George Clark, the maker, 

 dismounted and remounted it nine times. In every case, 

 different values were obtained in measuring the same space, 

 which in this instance was one-fourth of an inch. The differ- 

 ence between the greatest and the least values amounted to 

 over one two-thousand of an inch. 



This communication is already so long that I have room 

 only to add a word in reference to the unit recommended by 

 the late Congress at Indianapolis. The real unit must of 

 necessity be the meter. Granting this, it is simply a matter of 

 taste whether one shall say r mm., .1 cm., or .01 dm. It is, how- 

 ever, desirable to be uniform in our practice. I am decidedly 

 in favor of one centimeter as the unit, thus following more 

 closely the analogy of the English custom with respect to the 

 inch, but it will probably be better to adhere to the continental 

 practice if that can be ascertained. 



Harv.\rd College Observatory, December, 1878. 



Trichinae in Pork. Messrs, Belfield and Atwood, of Chicago, 

 have been engaged in making some examinations of the pork found 

 in the Chicago markets. The samples were furnished by the health 

 officers, and a report of their results was lately published in the local 

 papers. In the examination of one hundred specimens, eight were 

 found more or less infested with the parasite. These gentlemen have 

 also experimented with various agents with a view to destroying the 

 vitality of the worm during the process of packing the meat. Sul- 

 phurous acid appears to be very efficacious in killing the worms, and 

 may be found applicable for this purpose. 



