Besides the singing and calling, there is a peculiar sound which is only heard in summer. Waiting quietly to discover what birds are about, I become aware of a sound in the very air. It is not the midsummer hum which will soon be heard over the heated hay in the valley and over the cooler hills alike. It is not enough to be called a hum, and does but just tremble at the extreme edge of hearing. If the branches wave and rustle they overbear it; the buzz of a passing bee is so much louder it overcomes all of it that is in the whole field. I cannot, define it, except by calling the hours of winter to mind--they are silent; you hear a branch crack or creak as it rubs another in the wood, you hear the hoar-frost crunch on the grass beneath your feet, but the air is without sound in itself. The sound of summer is everywhere--in the passing breeze, in the hedge, in the broad branching trees, in the grass as it swings; all the myriad particles that together make the summer are in motion. The sap moves in the trees, the pollen is pushed out from grass and flower, and yet again these acres and acres of leaves and square miles of grass blades--for they would cover acres and square miles if reckoned edge to edge--are drawing their strength from the atmosphere. Exceedingly minute as these vibrations must be, their numbers perhaps may give them a volume almost reaching in the aggregate to the power of the ear. Besides the quivering leaf, the swinging grass, the fluttering bird's wing, and the thousand oval membranes which innumerable insects whirl about, a faint resonance seems to come from the very earth itself. The fervour of the sunbeams descending in a tidal flood rings on the strung harp of earth. It is this exquisite undertone, heard and yet unheard, which brings the mind into sweet accordance with the wonderful instrument of nature.