  The study of variations in solar activity is important for understanding the
underlying mechanism of solar activity and for predicting the level of activity
in view of the activity impact on space weather and global climate. Here we
have used the amplitudes (the peak values of the 13-month smoothed
international sunspot number) of Solar Cycles 1-24 to predict the relative
amplitudes of the solar cycles during the rising phase of the upcoming
Gleissberg cycle. We fitted a cosine function to the amplitudes and times of
the solar cycles after subtracting a linear fit of the amplitudes. The best
cosine fit shows overall properties (periods, maxima, minima, etc.) of
Gleissberg cycles, but with large uncertainties. We obtain a pattern of the
rising phase of the upcoming Gleissberg cycle, but there is considerable
ambiguity. Using the epochs of violations of the Gnevyshev-Ohl rule (G-O rule)
and the `tentative inverse G-O rule' of solar cycles during the period
1610-2015, and also using the epochs where the orbital angular momentum of the
Sun is steeply decreased during the period 1600-2099, we infer that Solar Cycle
25 will be weaker than Cycle 24. Cycles 25 and 26 will have almost same
strength, and their epochs are at the minimum between the current and upcoming
Gleissberg cycles. In addition, Cycle 27 is expected to be stronger than Cycle
26 and weaker than Cycle 28, and Cycle 29 is expected to be stronger than both
Cycles 28 and 30. The maximum of Cycle 29 is expected to represent the next
Gleissberg maximum. Our analysis also suggests a much lower value (30-40) for
the maximum amplitude of the upcoming Cycle 25.
