Holmes needed to know the elm tree's height to find where its shadow had fallen.
To find where the end of its shadow had fallen. When Holmes asked Musgrave about the height of the elm tree, he needed to know in order to determine where the end of its shadow had fallen.
Example: This is similar to using the height of a stick to find the length of its shadow on the ground.
Calculating the shadow's length from the height is a common practice in navigation and trigonometry.
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