I have started out reading seminal paper of Solow - Solow Growth model. It starts out with discussing weaknesses in Harrod Domar Model, a simple model of economic growth which featured prior to solow's work. You can read about Harrod model here
While discussing about Harrod Domar Model, Solow said it consistently uses short run tools for long run analysis (see the picture). But I don't understand why he called 'the multiplier' as a short run tool and 'margin(al) [product]' as a long run tool?