2$o Mr. Nicholson’s Method of graduating 
may be made of tranfparent horn or tortoife-fhelJ, which are 
in many refpecls more convenient than any opake fubAance. 
The Ufe. Apply the edge of either B or C to the confequent, 
and hide the piece A to the antecedent, obferving the difference 
between the numbers on the pieces denoting the lines they are 
found on ; then, if the fame edge of A be applied to any other 
antecedent, the other piece B or C, made ufe of, will inter- 
l’ecl a confequent in the fame ratio upon that line of the ar- 
rangement, which has the fame ftuation, with refpedt to the 
antecedent, as the line of the former confequent had to its ante- 
cedent. The numbers on the pieces ferve to indicate the rela- 
tive fituations. But if B be the confequent piece, and fall 
without the rule, the piece C will fbew the confequent one line 
lower ; or if C, in the like cafe, fall without the rule, B will 
fhew the confequent one line higher. It would be eafy to make 
the fame kind of provifion for the numbers which fall laterally 
without the rule : and it might be found convenient if, for the 
purpofe of computation, inftruments of this kind were to be 
made with an hundred or more lines. But in the prefent in- 
Arument, the numbers on the pieces will anfwer the fame 
purpofe : for if a confequent fall on a line at any given num- 
ber of intervals without the rule, it will be found on that line 
of the arrangement, which occupies the fame number of in- 
tervals, reckoned inwards from the oppofite edge of the rule. 
Fig. c. is a Gunter’s fcale, equivalent to that of 281 
inches in length, publifhed by the late Mr. Robertson. It 
is, however, but one-fourth of the length, and contains 
only one-foui th of the quantity of divilion. In the Aider 
GH is a moveable piece AB, acrofs which a fine line is 
drawn ; and there are alfo lines CD, EF, drawn acfofs the 
Aider, at a diftance from each other equal to the length of the 
rule. The ufe of this is fimilar to that of the foregoing. The 
line 
