On the evening of the twenty-fourth, after we had encamped, several bull bisons, being on the windward side, came so near us as to create a disturbance among our horses, who were not yet so familiarized to the formidable appearance of those animals, as to regard their near approach with indifference. The bulls at length became troublesome, approaching so near to smell at the horses, that some of the latter broke the cords by which they were fastened, and made their escape. A man was then sent to frighten away the bisons, who, in their turn, exhibited as much terror as they had occasioned to our horses. On Sunday, the twenty-fifth, we remained encamped, and some of the men were employed in drying a part of the meat killed on the preceding day. {163} This was done that we might be able to carry constantly with us a small supply of provisions, in reserve against any occasion when we might not meet with game. The magnetic variation equated from two sets of observations, was found to be 14° east. Observations for longitude were made; it was also attempted to take the meridian altitude of Antares for ascertaining the latitude, but the observation was commenced a few minutes too late, we having been longer occupied in making the preceding observations than we had anticipated.[115] [115] There is a considerable error in the longitude of points in western Nebraska and Colorado as given on the map of the expedition. For example, the longitude of the town of North Platte, at the confluence of the forks of the Platte, is 100° 45' 53"; while on the accompanying map the one hundred and second meridian runs but little west of the spot. For this reason, and because of the few landmarks described, the location of the camps of the expedition can only be approximated; but that of June 26 must have been quite near the Nebraska-Colorado boundary. The South Platte leaves Colorado almost exactly at the northeastern corner of the state.—ED. 26th. The weather had now been for some days fair. As we approached the mountains, we felt or fancied, a very manifest change in the character of the weather, and the temperature of the air. Mornings and evenings were usually calm, and the heat more oppressive than in the middle of the day. Early in the forenoon, a light and refreshing breeze often sprung up, blowing from the west or south-west, which again subsided on the approach of night. This phenomenon was so often observed that we were induced to attribute it to the operation of the same local cause, which in the neighbourhood of the sea, produces a diurnal change in the winds, which blow alternately to and from the shore. The Rocky Mountains may be considered as forming the shore of that sea of sand, which is traversed by the Platte, and extends northward to the Missouri, above the great bend.