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原文如下：
{{from|A<span style{{=}}"font-size: 70%;>RTICLE</span> (291.) T<span style{{=}}"font-size: 70%;>HE</span> Integral Calculus is the reverse of the Differential Calculus, its object being to determine the expression or function from which a given differential has been derived.<br><br>Thus we have found that the differential of ''x''<sup>2</sup>'' is 2''2xdxxdx'', therefore, if we have given 2''2xdxxdx'', we know that it must have been derived from ''x'', or plus a constant term.<br><br>(292.) '''The function from which the given differential has been derived, is called its ''integral''.''' Hence, as we are not certain whether the integral has a constant quantity or not added to it, we annex a constant quantity represented by C, the value of which is to be determined from the nature of the problem. <br><br>(293.) Leibnitz considered the differentials of functions as indefinitely small differences, and the sum of these indefinitely small differences he regarded as making up the function; hence the letter S was placed before the differential to show that the sum was to be taken. As it was frequently required to place S before a compound expression, it was elongated into the sign ''∫'', which, being placed before a differential, denotes that its integral is to be taken. Thus,<br><center><math>\int 2xdx = x^2+C</math>.</center>This sign ''∫'' is still retained ever by those whu reject the philosophy of Leibnitz. |''Elements of Analytical Geometry and of The Differential and Integral Calculus'', Integral Calculus, Section I}}
实际上，书中的积分号仅仅写作<span style="font-family:serif;">''∫''</span>，没有写得像今天的<math>\int</math>那么长。
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