This paper relates to the second theorem, viz. that which asserts that every number is composed of 4 square numbers (0 [or zero] being considered as an even square). If every odd number be composed of 4 square numbers, then every even number must also be composed of 4 square numbers; for every even number must, on a continued division by 2, ultimately become an odd number. The paper relates chiefly to the Table which accompanies it, from which it appears that a remarkable law obtains as to the division of odd numbers (2
n
+ 1) into 4 square numbers—when a number of the form 4
n
+ 1 is divisible into 2 square numbers, which (as 4
n
+ 1 is an odd number) must be one of them odd, the other even. Before explaining the Table, it is proper to state that if an odd number be divisible into 4 square numbers, three of them must be odd, and one of them even, or one of them must be odd, and 3 of them even, otherwise their sum cannot be an odd number; it follows from this that if the difference between any two of them be an odd number, the difference between the other two must be an even number, and
vice versâ
; for let
a
2
+
b
2
+
c
2
+
d
2
= 2
n
+ 1, then if
a
2
-
b
2
= 2
p
,
c
2
-
d
2
must equal 2
q
+ 1; if possible let
c
2
—
d
2
= 2
r
, then
a
2
-
b
2
+
c
2
-
d
2
= 2
p
+2
r
; add 2
b
2
+ 2
d
2
(an even number) to each, and
a
2
+
b
2
+
c
2
+
d
2
will be an even number, which by the hypothesis it is n o t; if, therefore,
a
2
—
b
2
be an even number,
c
2
—
d
2
cannot also be an even number, and therefore must be an odd one. If, therefore, the four roots of the squares into which any odd number may be divided are arranged in any order there will be three differences; the two exterior differences will be one odd, the other even; the middle difference may be either odd or even. The Table is arranged thus:—the lowest row of figures is the series 1, 5, 9, 13, 17, &c. (4
n
+ 1); the next row above is the series of natural numbers, 0, 1, 2, 3, 4, &c. (
n
), &c.; the next row is 1, 3, 5, 7, 9, &c. (2
n
+ 1) the odd numbers; each of the odd numbers is the first term in a series increasing upwards by the numbers 2, 4, 6, 8, 10, &c., forming an arithmetic series of the second order (the first and second differences being respectively 2 each); when the number in the lowest row cannot be divided into 2 squares the arithmetic series is not formed, and the squares are marked with an asterisk, but when the number 4
n
+ 1 is divisible into 2 square numbers, the roots of these squares constitute the two exterior differences of the roots into which the odd number may be divided, and also of the roots into which each term of the series increasing upward may be divided; the middle difference of the roots will be the smaller half of the sum of the 2 roots of the square numbers into which 4
n
+ 1 may be divided, with a negative sign, and will increase by 1 in each successive term of the upward series.