5. All your meteorologic instruments should be carefully compared throughout a large extent of the scales, and tabulated for the purpose of applying the requisite corrections when necessary, and one or more of them should be compared with the standard instruments at the Royal Society or Royal Observatory on your return home. 6. All observations which involve the comparison of minute differences should be the mean result of at least three readings, and should be as much as possible the province of the same individual observer. 7. In some of those singularly heavy showers which occur in crossing the Equator, and also at the changes of the Monsoon, attempts should be made to measure the quantity of rain that falls in a given time. A very rude instrument, if properly placed, will answer this purpose, merely a wide superficial basin to receive the rain, and to deliver it into a pipe, whose diameter, compared with that of the mouth of the basin, will show the number of inches, etc. that have fallen on an exaggerated scale. 8. It is unnecessary to call your attention to the necessity of recording every circumstance connected with that highly interesting phenomenon, the Aurora Australis, such as the angular bearing and elevation of the point of coruscation; the bearing also of the principal luminous arches, etc. 9. It has been asserted that lunar and solar halos are not always exactly circular, and a general order might, therefore, be given to the officer of the watch, to measure their vertical and horizontal diameters whenever they occur, day or night. Large collections of natural history cannot be expected, nor any connected account of the structure or geological arrangements of the great islands which you are to coast; nor, indeed, would minute inquiries on these subjects be at all consistent with the true objects of the survey. But, to an observant eye, some facts will unavoidably present themselves, which will be well worth recording, and the medical officers will, no doubt, be anxious to contribute their share to the scientific character of the survey. I have now exhausted every subject to which it can be necessary to call the attention of an officer of your long experience; and I have, therefore, only further to express my conviction, that if Providence permits you to retain your wonted health and activity, you will pursue the great objects of this expedition with all the energy in your power, and with all the perseverance consistent with a due regard to the safety of His Majesty's Ship, and to the comfort of your officers and crew. Given, etc. this 8th of June, 1837. F. Beaufort, Hydrographer. ... OFFICERS AND CREW. The crew embarked in the Beagle in this her third voyage, consisted of: