—The writer when introducing his pupils to the use of Logarithmic Tables is accustomed to give them exercise with the number 1·00023. This number, he tells them, without explaining how he obtains it, is such that 1·00023 × 1·00023 × … 10,000 times would give a, number which read to the 3rd decimal place is equal to 10. He makes this the basis of practical exercises in approximations, and builds on it lessons in Indices, and in the use of Logarithmic Tables, with particular explanation of the Mean Difference Columns.