| wikihow | "The Telescope in the Seventeenth Century" |
|---|---|
| com, Retrieved 28-3-2018 | 65 1 : 38—58 |
| Retrieved 2021-01-27 | 2: 153—154 |
|---|---|
| Hutchinson, D | While mariners benefited from the accurate charts, they could not receive telegraph signals while under way, and so could not use the method for navigation |
wikihow.
| Littlehales, G | wikihow |
|---|---|
| Howse, Derek 1980 | Philosophical Transactions |