S92 On an improved Cross for Land-survej/itig. 



ON MR. Newton's improved cross for land-surveying. 

 To the Editor of the Philosophical Magazine and Journal. 



Sir, 



The last Number of the Philosophical Magazine contains 

 some observations on Mr. Newton's improved cross for land- 

 surveyors. 



The points on the diagonal of a field where the true perpendi- 

 culars fiill are, I admit, as readily found by means of the old 

 cross as by means of the new one ; but the measuring of these 

 or any other straight lines is certainly more accurately and 

 more expeditiously accomplished by the latter than the former. 

 " Leander's" calculations, most probably through mere inad- 

 vertence, are the very reverse of what they ought to be. In- 

 stead of dividing the fraction i by the numbers 2, 3, 4, &c. he 

 ought to have multiplied by these numbers ; which would have 

 given, instead of y'^, jV' sV' ^^- ^'^^ numbers f , f , f , &c. 

 errors increasing in proportion to the length of the lines to be 

 measured. Now if this new instrument will enable the sur- 

 veyor without any additional trouble to correct or avoid these 

 errors, there is doubtless an advantage attending the use of it, 

 whatever may be the custom of the surveyor in " reading off" 

 his lines, &c. — " Leander's" method of measuring an inaccessi- 

 ble Ime appears to me very ingenious : at the same time I think 

 that when a " pond " only intercepts the points a and d, the 

 line da may be measured more readily as follows : — On the 

 same side o^ da take two equal perpendiculars a B, rf C to 

 da, so shall the distance from B to C be equal to da as was 

 requii'ed. I am, sir, &c. 



Wisbech, Nov. 1825. Y. Z. 



NOTICE ON MR. TREDGOLd's CORRECTION OF DR. URE's CAL- 

 CULATION OF THE LATENT HEAT OF VAPOURS. 



To the Editor of the Philosophical Magazine and Journal. 



Sir, 

 At page 277 of your last Number, Mr. Tredgold points out 

 " a considerable error in the mode of calculating the results of 

 the experiments " of Dr. Ure on the latent heat of vapours, 

 which, he says, " has not been noticed." Now, if your readers 

 will turn to Mr. Herapath's paper in the Annals of Philosophy 

 for December 1821, pp. 450 and 458, they will find the very 

 number 888° for aqueous vapour Mr. T. gives, with Dr. Ure's 

 errors, and the cause of them, particularly pointed out. In 

 Phil. Mag. for Oct. 1822, p. 299, they will also find these 

 same mistakes animadverted upon in a paper signed D. 

 I am, sir, yours, 8cc. 



Candour. 



