Mr. W. S. MacLeay in reply to Ruralis. 357 



Fahrenheit's scale at the time of taking its rate of expansion 

 by the boiling of mercury: and a Table of such values should 

 be furnished for each register by the maker of the instrument. 

 The following, for example, would be the proper Tab. ef or 

 register I, which has been so often referred to, in which the 

 arc for the boiling of mercury or 600° (without adding the 

 initial temperature) was 1 ° 20'. 



Table XII. 



^ Expansion. Temperature. o , Expansion. Temperature. 



1 = -00872 = 450 

 30 = -00436 = 225 

 20 = -00290 = 150 

 \5 = -00218 = 112 



With such a Table an intelligent workman could employ 

 the instrument without any material error. Those who might 

 object to the expense of a platinum bar may substitute an iron 

 one for ordinary purposes, and the cost of the black-lead re- 

 gister can never be an obstacle to its general use. Other sub- 

 stances might obviously be employed in its construction, but 

 the facility with which it can be worked, its small expansion, 

 its infusibility, and the impunity with which it bears the most 

 sudden changes of temperature (as when red hot it may even 

 be quenched in water without injury), will probably always 

 give the black-lead ware the preference. The only precaution 

 to be taken with it is to expose it previously, out of the con- 

 tact of air, to a heat at least as great as that in which it is in- 

 tended to employ the instrument 



XLV. Correction of a Quotation in a Paper " On the Impedi- 

 ments to the Study of Natural History," published in the 

 Phil. Mag. and Annals for May, 1831. % W. S. MacLeay, 

 Esq., M.A. F.L.S. 8sc. 



To Richard Taylor, Esq. Sfc. 8fC, 

 Dear Sir, 

 "VT'OUR Correspondent who signs himself " .RwraZ/s," has 

 -*■ in the Phil. Mag. and Annals for May last given us a me- 

 lancholy picture of " the Impediments to the Study of Natural 

 History." In his list of them, however, I think he has omitted 

 to make mention of that impediment which I fear has most 

 thwarted his own progress in the science. But this is of the 

 slightest consecjuence; and indeed were it all, I would not have 

 troubled you on the present occasion. But I must beg of your 

 Coi re.spon(lent that in future, when he does me the honour to 

 quote tVoin any work of mine, he will cite the exact words, 

 and not make me the author of downright nonsense. I have 

 nowlierc said that " the discovery of a natural system, being 



the 



